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CLASSICS 

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THE HARVARD CLASSICS 



The Five-Foot Shelf of Books 



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THE HARVARD CLASSICS 
EDITED BY CHARLES W. ELIOT, LL.D. 



Scientific Papers 

Physics • Chemistry 
Astronomy • Geology 

W//A Introductions and 'Notes 
Volume 30 




P. F. Collier & Son Corporation 

NEW YORK 



Copyright, 1910 
By p. F. Collier & Son 

manufactured in u. s. a. 



CONTENTS 

THE FORCES OF MATTER 

Delivered before a Juvenile Auditory at the Royal Institution 
OF Great Britain during the Christmas Holidays of 1859-60 

PAGE 

Lecture I. — The Force of Gravitation 7 

Lecture IL — Gravitation — Cohesion 22 

Lecture IIL — Cohesion — Chemical Affinity 37 

Lecture IV. — Chemical Affinity — Heat 50 

Lecture V. — Magnetism — Electricity 61 

Lecture VI. — The Correlation of the Physical Forces 73 

THE CHEMICAL HISTORY OF A CANDLE 

Lecture I. — A Candle: The Flame — Its Sources — Structure 

— Mobility — Brightness 86 

Lecture II. — A Candle: Brightness of the Flame — Air Neces- 
sary for Combustion — Production of Water .... loi 

Lecture III. — Products: Water from the Combustion — Na- 
ture OF Water — A Compound — Hydrogen 113 

Lecture IV. — Hydrogen in the Candle — Burns into Water — 
The Other Part of Water — Oxygen 128 

Lecture V. — Oxygen Present in the Air — Nature of the 
Atmosphere — Its Properties — Other Products from the 
Candle — Carbonic Acid — Its Properties 141 

Lecture VI. — Carbon or Charcoal — Coal-Gas — Respiration 

and its Analogy to the Burning of a Candle — Conclusion 156 

ON THE CONSERVATION OF FORCE 

Introduction to a Series of Lectures Delivered at Carlsruhe 

in the Winter of i 862-1 863 173 



2 CONTENTS 

ICE AND GLACIERS 

PAGE 

A Lecture Delivered at Frankfort-on-the-Main, and at 

Heidelberg, in February, 1865 211 

THE WAVE THEORY OF LIGHT 

[A Lecture Delivered at the Academy of Music, Philadelphia, 
Under the Auspices of the Franklin Institute, September 
29TH, 1884] 251 

THE TIDES 

[Evening Lecture to the British Association at the Southamp- 
ton Meeting, Friday, August 25TH, 1882] 274 

THE EXTENT OF THE UNIVERSE 311 

GEOGRAPHICAL EVOLUTION 325 



THE FORCES OF MATTER 
THE CHEMICAL HISTORY OF A CANDLE 



BY 

MICHAEL FARADAY 



INTRODUCTORY NOTE 

Michael Faraday was the son of a blacksmith, and was born at New- 
ington Butts, near London, September 22, 1791. He began life as an 
errand boy to a bookbinder and stationer, to whom he was later bound 
apprentice. After eight years in this business, he was engaged by Sir 
Humphry Davy as his laboratory assistant at the Royal Institution, and 
in 1813-15 he traveled extensively on the Continent with his master, and 
saw some of the most famous scientists of Europe. Shortly after his return 
to the Royal Institution, he began to make contributions of his own to 
science, his first paper appearing in 1816. He became director of the 
laboratory in 1825, and professor of chemistry in 1833; rising rapidly, 
through the number and importance of his discoveries, to a most dis- 
tinguished position. But he was working at too great pressure, and in 
1 84 1 his health gave way, so that for some three years he could not work 
at all. He recovered, however, and made some of his most important 
discoveries after this interruption; and was offered, but declined, the 
presidency of both the Royal Society and the Royal Institution. He died 
August 25, 1867. 

It was characteristic of Faraday's devotion to the enlargement of the 
bounds of human knowledge that on his discovery of magneto-electricity 
he abandoned the commercial work by which he had added to his small 
salary, in order to reserve all his energies for research. This financial 
loss was in part made up later by a pension of /300 a year from the 
British Government. 

Faraday's parents were members of the obscure religious denomination 
of the Sandemanians, and Faraday himself, shortly after his marriage, at 
the age of thirty, joined the same sect, to which he adhered till his death. 
Religion and science he kept strictly apart, believing that the data of 
science were of an entirely different nature from the direct communica- 
tions between God and the soul on which his religious faith was based. 

The discoveries made by Faraday were so numerous, and often demand 
so detailed a knowledge of chemistry and physics before they can be 
understood, that it is impossible to attempt to describe or even enumerate 
them here. Among the most important are the discovery of magneto- 
electric induction, of the law of electro-chemical decomposition, of the 
magnetization of light, and of diamagnetism. Round each of these are 
grouped numbers of derivative but still highly important additions to 
scientific knowledge, and together they form so vast an achievement as 



6 INTRODUCTORY NOTE 

to lead his successor, Tyndall, to say, "Taking him for all and all, I think 
it will be conceded that Michael Faraday was the greatest experimental 
philosopher the world has ever seen; and I will add the opinion, that the 
progress of future research will tend, not to dim or to diminish, but to 
enhance and glorify the labours of this mighty investigator." 

In spite of the highly technical nature of his work in research, Faraday 
was remarkably gifted as an expounder of science to popular audiences; 
and his lectures at the Royal Institution, especially those to younger audi- 
ences, were justly famous. The following examples are classics in the 
department of clear and fascinating scientific exposition. 



THE FORCES OF MATTER 

DELIVERED BEFORE A JUVENILE AUDITORY AT THE 
ROYAL INSTITUTION OF GREAT BRITAIN 
DURING THE CHRISTMAS HOLI- 
DAYS OF 1859-60 

By Michael Faraday 

LECTURE I 
THE FORCE OF GRAVITATION 

IT grieves me much to think that I may have been a cause of 
disturbance to your Christmas arrangements,* for nothing is 
more satisfactory to my mind than to perform what I under- 
take; but such things are not always left to our own power, and we 
must submit to circumstances as they are- appointed. I will to-day 
do my best, and will ask you to bear with me if I am unable to give 
more than a few words; and, as a substitute, I will endeavor to 
make the illustrations of the sense I try to express as full as possible; 
and if we find by the end of this lecture that we may be justified in 
continuing them, thinking that next week our power shall be greater, 
why then, with submission to you, we will take such course as you 
may think fit, either to go on or discontinue them; and although I 
now feel much weakened by the pressure of the illness (a mere cold) 
upon me, both in facility of expression and clearness of thought, I 
shall here claim, as I always have done on these occasions, the right 
of addressing myself to the younger members of the audience; and 
for this purpose, therefore, unfitted as it may seem for an elderly, 
infirm man to do so, I will return to second childhood, and become, 
as it were, young again among the young. 

Let us now consider, for a little while, how wonderfully we stand 
upon this world. Here it is we are born, bred, and live, and yet we 
*The opening lecture was twice postponed on account of Dr. Faraday's illness. 

7 



8 FARADAY 

view these things with an almost entire absence of wonder to our- 
selves respecting the way in which all this happens. So small, indeed, 
is our wonder, that we are never taken by surprise; and I do think 
that, to a young person of ten, fifteen, or twenty years of age, per- 
haps the first sight of a cataract or a mountain would occasion him 
more surprise than he had ever felt concerning the means of his own 
existence; how he came here; how he lives; by what means he stands 
upright; and through what means he moves about from place to 
place. Hence, we come into this world, we live, and depart from it, 
without our thoughts being called specifically to consider how all this 
takes place; and were it not for the exertions of some few inquiring 
minds, who have looked into these things, and ascertained the very 
beautiful laws and conditions by which we do live and stand upon 
the earth, we should hardly be aware that there was any thing 
wonderful in it. These inquiries, which have occupied philosophers 
from the earliest days, when they first began to find out the laws by 
which we grow, and exist, and enjoy ourselves, up to the present 
time, have shown us that all this was effected in consequence of the 
existence of certain forces, or abilities to do things, or powers, that 
are so common that nothing can be more so; for nothing is com- 
moner than the wonderful powers by which we are enabled to stand 
upright: they are essential to our existence every moment. 

It is my purpose to-day to make you acquainted with some of these 
powers; not the vital ones, but some of the more elementary, and 
what we call physical powers; and, in the outset, what can I do to 
bring to your minds a notion of neither more nor less than that 
which I mean by the word power or force? Suppose I take this 
sheet of paper, and place it upright on one edge, resting against a 
support before me (as the roughest possible illustration of something 
to be disturbed), and suppose I then pull this piece of string which 
is attached to it. I pull the paper over. I have therefore brought 
into use a power of doing so — the power of my hand carried on 
through this string in a way which is very remarkable when we 
come to analyze it; and it is by means of these powers conjointly 
(for there are several powers here employed) that I pull the paper 
over. Again, if I give it a push upon the other side, I bring into 
play a power, but a very different exertion of power from the former; 



THE FORCE OF GRAVITATION 9 

or, if I take now this bit of shell-lac [a stick of shell-lac about 12 
inches long and i 1-2 in diameter], and rub it with flannel, and 
hold it an inch or so in front of the upper part of this upright sheet, 
the paper is immediately moved towards the shell-lac, and by now 
drawing the latter away, the paper falls over without having been 
touched by any thing. You see, in the first illustration I produced 
an effect than which nothing could be commoner; I pull it over 
now, not by means of that string or the pull of my hand, but by some 
action in this shell-lac. The shell-lac, therefore, has a power where- 
with it acts upon the sheet of paper; and, as an illustration of the 
exercise of another kind of power, I might use gunpowder with 
which to throw it over. 

Now I want you to endeavor to comprehend that when I am 
speaking of a power or force, I am speaking of that which I used just 
now to pull over this piece of paper. I will not embarrass you at 
present with the name of that power, but it is clear there was a 
something in the shell-lac which acted by attraction, and pulled the 
paper over; this, then, is one of those things which we call power, or 
jorce; and you will now be able to recognize it as such in whatever 
form I show it to you. We are not to suppose that there are so very 
many different powers; on the contrary, it is wonderful to think 
how few are the powers by which all the phenomena of nature are 
governed. There is an illustration of another kind of power in that 
lamp; there is a power of heat — a power of doing something, but 
not the same power as that which pulled the paper over; and so, by 
degrees, we find that there are certain other powers (not many) in 
the various bodies around us; and thus, beginning with the simplest 
experiments of pushing and pulling, I shall gradually proceed to 
distinguish these powers one from the other, and compare the way 
in which they combine together. This world upon which we stand 
(and we have not much need to travel out of the world for illustra- 
tions of our subject; but the mind of man is not confined like the 
matter of his body, and thus he may and does travel outward, for 
wherever his sight can pierce, there his observations can penetrate) 
is pretty nearly a round globe, having its surface disposed in a 
manner of which this terrestrial globe by my side is a rough model; 
so much is land and so much is water, and by looking at it here we 



10 FARADAY 

see in a sort of map or picture how the world is formed upon its 
surface. Then, when we come to examine farther, I refer you to this 
sectional diagram of the geological strata of the earth, in which 
there is a more elaborate view of what is beneath the surface of our 
globe. And, when we come to dig into or examine it (as man does 
for his own instruction and advantage, in a variety of ways), we 
see that it is made up of different kinds of matter, subject to a 
very few powers; and all disposed in this strange and wonderful 
way, which gives to man a history — and such a history — as to what 
there is in those veins, in those rocks, the ores, the water-springs, the 
atmosphere around, and all varieties of material substances, held 
together by means of forces in one great mass, 8,000 miles in diameter, 
that the mind is overwhelmed in contemplation of the wonderful 
history related by these strata (some of which are fine and thin like 
sheets of paper), all formed in succession by the forces of which I 
have spoken. 

I now shall try to help your attention to what I may say by 
directing, to-day, our thoughts to one kind of power. You see what 
I mean by the term matter — any of these things that I can lay hold 
of with the hand, or in a bag (for I may take hold of the air by 
inclosing it in a bag) — they are all portions of matter with which we 
have to deal at present, generally or particularly, as I may require 
to illustrate my subject. Here is the sort of matter which we call 
water — it is there ice [pointing to a block of ice upon the table], 
there water — [pointing to the water boiling in a flask] — here vapor — 
you see it issuing out from the top [of the flask]. Do not suppose 
that that ice and that water are two entirely different things, or that 
the steam rising in bubbles and ascending in vapor there is absolutely 
different from the fluid water: it may be different in some par- 
ticulars, having reference to the amounts of power which it con- 
tains; but it is the same, nevertheless, as the great ocean of water 
around our globe, and I employ it here for the sake of illustration, 
because if we look into it we shall find that it supplies us with 
examples of all the powers to which I shall have to refer. For 
instance, here is water — it is heavy; but let us examine it with regard 
to the amount of its heaviness or its gravity. I have before me a 
little glass vessel and scales [nearly equipoised scales, one of which 



THE FORCE OF GRAVITATION II 

contained a half-pint glass vessel], and the glass vessel is at present 
the lighter of the tw^o; but if I now take some water and pour it in, 
you see that that side of the scales immediately goes down; that 
shows you (using common language, which I will not suppose for 
the present you have hitherto applied very strictly) that it is heavy, 
and if I put this additional weight into the opposite scale, I should 
not wonder if this vessel would hold water enough to weigh it 
down. [The lecturer poured more water into the jar, which again 
went down.] Why do I hold the bottle above the vessel to pour the 
water into it.'' You will say, because experience has taught me that it 
is necessary. I do it for a better reason — because it is a law of nature 
that the water should fall toward the earth, and therefore the very 
means which I use to cause the water to enter the vessel are those 
which will carry the whole body of water down. That power is 
what we call gravity, and you see there [pointing to the scales] a 
good deal of water gravitating toward the earth. Now here [ex- 
hibiting a small piece of platinum(')] is another thing which 
gravitates toward the earth as much as the whole of that water. See 
what a little there is of it; that little thing is heavier than so much 
water [placing the metal in opposite scales to the water]. What a 
wonderful thing it is to see that it requires so much water as that 
[a half-pint vessel full] to fall toward the earth, compared with the 
little mass of substance I have herel And again, if I take this metal 
[a bar of aluminium(^) about eight times the bulk of the platinum], 
we find the water will balance that as well as it did the platinum; 
so that we get, even in the very outset, an example of what we want 
to understand by the words forces or powers, 

I have spoken of water, and first of all of its property of falling 
downward : you know very well how the oceans surround the globe 
— how they fall round the surface, giving roundness to it, clothing 
it like a garment; but, besides that, there are other properties of 
water. Here, for instance, is some quicklime, and if I add some water 
to it, you will find another power and property in the water. (^) It is 

^Platinum, with one exception the heaviest body known, is 2i!4 times heavier than 
water. ^ Aluminium is 2 Yi times heavier than water. 

' Power or property in water. This power — the heat by which the water is kept 
in a jiuid state — is said, under ordinary circumstances, to be latent or insensible. When, 
however, the water changes its form, and, by uniting with the lime or sulphate of 
copper, becomes solid, the heat which retained it in a liquid state is evolved. 



12 FARADAY 

now very hot; it is steaming up; and I could perhaps light phos- 
phorus or a lucifer-match with it. Now that could not happen 
without a force in the water to produce the result; but that force 
is entirely distinct from its power of falling to the earth. Again, 
here is another substance [some anhydrous sulphate of copperC)] 
which will illustrate another kind of power. [The lecturer here 
poured some water over the white sulphate of copper, which imme- 
diately became blue, evolving considerable heat at the same time.] 
Here is the same water with a substance which heats nearly as much 
as the lime does, but see how differently. So great indeed is this 
heat in the case of lime, that it is sufficient sometimes (as you see 
here) to set wood on fire; and this explains what we have sometimes 
heard, of barges laden with quicklime taking fire in the middle of 
the river, in consequence of this power of heat brought into play 
by a leakage of the water into the barge. You see how strangely 
different subjects for our consideration arise when we come to think 
over these various matters — the power of heat evolved by acting upon 
lime with water, and the power which water has of turning this salt 
of copper from white to blue. 

I want you now to understand the nature of the most simple 
exertion of this power of matter called weight or gravity. Bodies 
are heavy; you saw that in the case of water when I placed it in the 
balance. Here I have what we call a weight [an iron half cwt.] — a 
thing called a weight because in it the exercise of that power of 
pressing downward is especially used for the purposes of weighing; 
and I have also one of these little inflated India-rubber bladders, 
which are very beautiful although very common (most beautiful 
things are common), and I am going to put the weight upon it, to 
give you a sort of illustration of the downward pressure of the iron, 
and of the power which the air possesses of resisting that pressure; 
it may burst, but we must try to avoid that. [During the last few 
observations the lecturer had succeeded in placing the half cwt. in 
a state of quiescence upon the inflated India-rubber ball, which 
consequently assumed a shape very much resembling a flat cheese 
with round edges.] There you see a bubble of air bearing half a 

* Anhydrous sulphate of copper: sulphate of copper deprived of its water of crystal- 
lization. To obtain it the blue sulphate is calcined in an earthen crucible. 



THE FORCE OF GRAVITATION I3 

hundred-weight, and you must conceive for yourselves what a won- 
derful power there must be to pull this weight downward, to sink 
it thus in the ball of air. 

Let me now give you another illustration of this power. You 
know what a pendulum is. I have one here (Fig. 1), and if I set it 
swinging, it will continue to swing to and 
fro. Now I wonder whether you can tell me 
why that body oscillates to and fro — that J 

pendulum bob, as it is sometimes called. / 

Observe, if I hold the straight stick horizon- / 

tally, as high as the position of the ball at the / 

two ends of its journey, you see that the ball ^ 

is in a higher position at the two extremities 
than it is when in the middle. Starting from 
one end of the stick, the ball falls toward the centre, and then rising 
again to the opposite end, it constantly tries to fall to the lowest point, 
swinging and vibrating most beautifully, and with wonderful prop- 
erties in other respects — the time of its vibration, and so on — but 
concerning which we will not now trouble- ourselves. 

If a gold leaf, or piece of thread, or any other substance were hung 
where this ball is, it would swing to and fro in the same manner, 
and in the same time too. Do not be startled at this statement; I 
repeat, in the same manner and in the same time, and you will see 
by-and-by how this is. Now that power which caused the water to 
descend in the balance — which made the iron weight press upon and 
flatten the bubble of air — which caused the swinging to and fro o£ 
the pendulum, that power is entirely due to the attraction which 
there is between the falling body and the earth. Let us be slow and 
careful to comprehend this. It is not that the earth has any particular 
attraction toward bodies which fall to it, but, that all these bodies 
possess an attraction every one toward the other. It is not that the 
earth has any special power which these balls themselves have not; 
for just as much power as the earth has to attract these two balls 
[dropping two ivory balls], just so much power have they in pro- 
portion to their bulks to draw themselves one to the other; and the 
only reason why they fall so quickly to the earth is owing to its 
greater size. Now if I were to place these two balls near together, 



14 FARADAY 

I should net be able, by the most delicate arrangement of apparatus, 
to make you, or myself, sensible that these balls did attract one 
another; and yet we know that such is the case, because if, instead 
of taking a small ivory ball, we take a mountain, and put a ball 
like this near it, we find that, owing to the vast size of the mountain 
as compared with the billiard ball, the latter is drawn slightly toward 
it, showing clearly that an attraction does exist, just as it did between 
the shell-lac which I rubbed and the piece of paper which was over- 
turned by it. 

Now it is not very easy to make these things quite clear at the 
outset and I must take care not to leave anything unexplained as I 
proceed, and, therefore, I must make you clearly understand that all 
bodies are attracted to the earth, or, to use a more learned term, 
gravitate. You will not mind my using this word, for when I say 
that this penny-piece gravitates, I mean nothing more nor less than 
that it falls toward the earth, and, if not intercepted, it would go on 
falling, falling, until it arrived at what we call the centre of gravity 
of the earth, which I will explain to you by-and-by. 

I want you to understand that this property of gravitation is never 
lost; that every substance possesses it; that there is never any change 
in the quantity of it; and, first of all, I will take as illustration a 
piece of marble. Now this marble has weight, as you will see if I put 
it in these scales; it weighs the balance down, and if I take it off, 
the balance goes back again and resumes its equilibrium. I can 
decompose this marble and change it in the same manner as I can 
change ice into water and water into steam. I can convert a part of 
it into its own steam easily, and show you that this steam from the 
marble has the property of remaining in the same place at common 
temperatures, which water steam has not. If I add a little liquid to 
the marble and decompose it Q), I get that which you see — [the 
lecturer here put several lumps of marble into a glass jar, and poured 

* Add a little liquid to the marble and decompose it. Marble is composed of carbonic 
acid and lime, and, in chemical language, is called carbonate of lime. When sulphuric 
acid is added to it, the carbonic acid is set free, and the sulphuric acid unites with the 
lime to form sulphate of lime. 

Carbonic acid, under ordinary circumstances, is a colorless invisible gas, about half 
as heavy again as air. Dr. Faraday first showed that under great pressure it could be 
obtained in a liquid state. Thilorier, a French chemist, afterward found that it could 
be sdidiiied. 



THE FORCE OF GRAVITATION 1 5 

water and then acid over them; the carbonic acid immediately com- 
menced to escape with considerable effervescence] — the appearance 
of boiling, which is only the separation of one part of the marble 
from another. Now this [marble] steam, and that [water] steam, 
and all other steams, gravitate just like any other substance does; 
they all are attracted the one toward the other, and all fall toward the 
earth, and what I want you to see is that this steam gravitates. I 
have here (Fig. 2) a large vessel placed upon a balance, and the 




Fig. 2 

moment I pour this steam into it you see that the steam gravitates. 
Just watch the index, and see whether it tilts over or not. [The 
lecturer here poured the carbonic acid out of the glass in which it 
was being generated into the vessel suspended on the balance, when 
the gravitation of the carbonic acid was at once apparent.] Look 
how it is going down. How pretty that is! I poured nothing in but 
the invisible steam, or vapor, or gas which came from the marble, 
but you see that part of the marble, although it has taken the shape 
of air, still gravitates as it did before. Now will it weigh down that 
bit of paper? [placing a piece of paper in the opposite scale.] Yes, 
more than that; it nearly weighs down this bit of paper [placing 
another piece of paper in]. And thus you see that other forms of 
matter besides solids and liquids tend to fall to the earth; and, 
therefore, you will accept from me the fact that all things gravitate, 
whatever may be their form or condition. Now here is another 
chemical test which is very readily applied. [Some of the carbonic 
acid was poured from one vessel into another, and its presence in 
the latter shown by introducing into it a lighted taper, which was 
immediately extinguished.] You see from this result also that it 



1 6 FARADAY 

gravitates. All these experiments show you that, tried by the balance, 
tried by pouring like water from one vessel to another, this steam, 
or vapor, or gas is, like all other things, attracted to the earth. 

There is another point I want in the next place to draw your 
attention to. I have here a quantity of shot; each of these falls 
separately, and each has its own gravitating power, as you perceive 
when I let them fall loosely on a sheet of paper. If I put them into a 
bottle, I collect them together as one mass, and philosophers have 
discovered that there is a certain point in the middle of the whole 
collection of shots that may be considered as the one point in which 
all their gravitating power is centred, and that point they call the 
centre of gravity; it is not at all a bad name, and rather a short one — 
the centre of gravity. Now suppose I take a sheet of pasteboard, or 
any other thing easily dealt with, and run a bradawl through it at 





Fig. 3 

one corner, A (Fig. 3), and Mr. Anderson holds that up in his hand 
before us, and I then take a piece of thread and an ivory ball, and 
hang that upon the awl, then the centre of gravity of both the paste- 
board and the ball and string are as near as they can get to the 
centre of the earth; that is to say, the whole of the attracting power 
of the earth is, as it were, centred in a single point of the cardboard, 
and this point is exactly below the point of suspension. All I have 
to do, therefore, is to draw a line, A B, corresponding with the 
string, and we shall find that the centre of gravity is somewhere in 



THE FORCE OF GRAVITATION 1 7 

that line. But where ? To find that out, all we have to do is to take 
another place for the awl (Fig. 4) , hang the plumb-line, and make 
the same experiment, and there [at the point C] is the centre of 
gravity, — there where the two lines which I have traced cross each 
other; and if I take that pasteboard and make a hole with the brad- 
awl through it at that point, you will see that it will be supported 
in any position in which it may be placed. Now, knowing that, 
what do I do when I try to stand upon one leg.? Do you not see 
that I push myself over to the left side, and quietly take up the 
right leg, and thus bring some central point in my body over this 
left leg? What is that point which I throw over.? You will know 
at once that it is the centre of gravity — that point in me where the 
whole gravitating force of my body is centred, and which I thus 
bring in a line over my foot. 

Here is a toy I happened to see the other day, which will, I think, 
serve to illustrate our subject very well. That toy ought to lie some- 
thing in this manner (Fig. 5), and would do so if it were uniform 
in substance; but you see it does not; it will get up again. And now 




Fig. 5 



Fig. 6 



philosophy comes to our aid, and I am perfectly sure, without look- 
ing inside the figure, that there is some arrangement by which the 
centre of gravity is at the lowest point when the image is standing 
upright; and we may be certain, when I am tilting it over (see 
Fig. 6), that I am Ufting up the centre of gravity (a), and raising 
it from the earth. All this is effected by putting a piece of lead 
inside the lower part of the image, and making the base of large 
curvature, and there you have the whole secret. But what will 



1 8 FARADAY 

happen if I try to make the figure stand upon a sharp point? You 
observe I must get that point exactly under the centre of gravity, or 
it vv^ill fall over thus [endeavoring unsuccessfully to balance it] ; and 

this, you see, is a difficult matter; I can 
not make it stand steadily; but if I 
embarrass this poor old lady with a 
world of trouble, and hang this wire 
with bullets at each end about her 
neck, it is very evident that, owing to 
there being those balls of lead hanging 
down on either side, in addition to the 
lead inside, I have lowered the centre 
of gravity, and now she will stand 
upon this point (Fig. 7), and, what is 
more, she proves the truth of our phi- 
losophy by standing sideways. 

I remember an experiment which 
puzzled me very much when a boy. I 
read it in a conjuring book, and this 
was how the problem was put to us: "How," as the book said, "how 
to hang a pail of water, by means of a stick, upon the side of a table" 
(Fig. 8). Now I have here a table, a piece of stick, and a pail, and the 




Fig. 7 




Fig. 8 Fig. 9 

proposition is, how can that pail be hung to the edge of this table ? It 
is to be done, and can you at all anticipate what arrangement I shall 
make to enable me to succeed ? Why this. I take a stick, and put it in 



THE FORCE OF GRAVITATION I9 

the pail between the bottom and the horizontal piece of wood, and 
thus give it a stiff handle, and there it is; and, what is more, the more 
water I put into the pail, the better it will hang. It is very true that 
before I quite succeeded I had the misfortune to push the bottoms 
of several pails out; but here it is hanging firmly (Fig. 9), and you 
now see how you can hang up the pail in the way which the con- 
juring books require. 

Again, if you are really so inclined (and I do hope all of you are), 
you will find a great deal of philosophy in this [holding up a cork 
and a pointed thin stick about a foot 
long]. Do not refer to your toy-books, 
and say you have seen that before. An- 
swer me rather, if I ask you, have you 
understood it before? It is an experiment 
which appeared very wonderful to me 
when I was a boy. I used to take a piece 
of cork (and I remember I thought at 
first that it was very important that it 
should be cut out in the shape of a man, 
but by degrees I got rid of that idea), and 
the problem was to balance it on the 
point of a stick. Now you will see I have only to place two sharp- 
pointed sticks one on each side, and give it wings, thus, and you will 
find this beautiful condition fulfilled. 

We come now to another point. All bodies, whether heavy or 
light, fall to the earth by this force which we call gravity. By obser- 
vation, moreover, we see that bodies do not occupy the same time in 
falling; I think you will be able to see that this piece of paper and 
that ivory ball fall with different velocities to the table [dropping 
them] ; and if, again, I take a feather and an ivory ball, and let them 
fall, you see they reach the table or earth at different times; that is 
to say, the ball falls faster than the feather. Now that should not be 
so, for all bodies do fall equally fast to the earth. There are one or 
two beautiful points included in that statement. First of all, it is 
manifest that an ounce, or a pound, or a ton, or a thousand tons, all 
fall equally fast, no one faster than another: here are two balls of 
lead, a very light one and a very heavy one, and you perceive they 




Fig. 10 



20 FARADAY 

both fall to the earth in the same time. Now if I were to put into a 
little bag a number of these balls sufficient to make up a bulk equal 
to the large one, they would also fall in the same time; for if an 
avalanche fall from the mountains, the rocks, snow, and ice, together 
falling toward the earth, fall with the same velocity, whatever be 
their size. 

I can not take a better illustration of this than that of gold leaf, 
because it brings before us the reason of this apparent difference in 
the time of the fall. Here is a piece of gold leaf. Now if I take a 
lump of gold and this gold leaf, and let them fall through the air 
together, you see that the lump of gold — the sovereign or coin — will 
fall much faster than the gold leaf. But why ? They are both gold, 
whether sovereign or gold leaf. Why should they not fall to the 
earth with the same quickness? They would do so, but that the air 
around our globe interferes very much where we have the piece of 
gold so extended and enlarged as to offer much obstruction on falling 
through it. I will, however, show you that gold leaf does fall as fast 
when the resistance of the air is excluded; for if I take a piece of 
gold leaf and hang it in the centre of a bottle so that the gold, and 
the bottle, and the air within shall all have an equal chance of falling, 
then the gold leaf will fall as fast as anything else. And if I suspend 
the bottle containing the gold leaf to a string, and set it oscillating 
like a pendulum, I may make it vibrate as hard as I please and the 
gold leaf will not be disturbed, but will swing as steadily as a piece 
of iron would do; and I might even swing it round my head with 
any degree of force, and it would remain undisturbed. Or I can 
try another kind of experiment: if I raise the gold leaf in this way 
[pulling the bottle up to the ceiling of the theatre by means of a 
cord and pulley, and then suddenly letting it fall within a few 
inches of the lecture table], and allow it then to fall from the ceiling 
downward (I will put something beneath to catch it, supposing I 
should be maladroit), you will perceive that the gold leaf is not in 
the least disturbed. The resistance of the air having been avoided, 
the glass bottle and gold leaf all fall exactly in the same time. 

Here is another illustration: I have hung a piece of gold leaf in 
the upper part of this long glass vessel, and I have the means by a 
little arrangement at the top, of letting the gold leaf loose. Before 



THE FORCE OF GRAVITATION 21 

we let it loose we will remove the air by means of an air-pump, 
and, while that is being done, let me show you another experiment 
of the same kind. Take a penny-piece, or a half crown, and a round 
piece of paper a trifle smaller in diameter than the coin, and try 
them side by side to see whether they fall at the same time [dropping 
them]. You see they do not — the penny-piece goes down first. But, 
now place this paper flat on the top of the coin, so that it shall not 
meet with any resistance from the air, and upon then dropping them 
you see they do both fall in the same time [exhibiting the effect]. I 
dare say, if I were to put this piece of gold leaf, instead of the paper, 
on the coin, it would do as well. It is very difficult to lay the gold leaf 
so flat that the air shall not get under it and lift it up in falling, 
and I am rather doubtful as to the success of this, because the gold 
leaf is puckery, but will risk the experiment. There they go together! 
[letting them fall] and you see at once that they both reach the 
table at the same moment. 

We have now pumped the air out of the vessel, and you will per- 
ceive that the gold leaf will fall as quickly in this vacuum as the 
coin does in the air. I am now going to let it loose, and you must 
watch to see how rapidly it falls. There! [letting the gold loose] 
there it is, falling as gold should fall. 

I am sorry to see our time for parting is drawing so near. As we 
proceed, I intend to write upon the board behind me certain words, 
so as to recall to your minds what we have already examined; and 
I put the word Forces as a heading, and I will then add beneath the 
names of the special forces according to the order in which we con- 
sider them; and although I fear that I have not sufficiently pointed 
out to you the more important circumstances connected with the 
force of Gravitation, especially the law which governs its attraction 
(for which, I think, I must take up a little time at our next meeting), 
still I will put that word on the board, and hope you will now 
remember that we have in some degree considered the force of 
gravitation — that force which causes all bodies to attract each other 
when they are at sensible distances apart, and tends to draw them 
together. 



LECTURE II 
GRAVITATION— COHESION 

DO me the favor to pay me as much attention as you did at 
our last meeting, and I shall not repent o£ that which I 
have proposed to undertake. It will be impossible for us 
to consider the Laws of Nature, and what they effect, unless we now 
and then give our sole attention, so as to obtain a clear idea upon the 
subject. Give me now that attention, and then I trust we shall not 
part without our knowing something about those laws, and the 
manner in which they act. You recollect, upon the last occasion, I 
explained that all bodies attracted each other, and that this power we 
called gravitation. I told you that when we brought these two bodies 
[two equal-sized ivory balls suspended by threads] near together, 
they attracted each other, and that we might suppose that the whole 
power of this attraction was exerted between their respective centres 
of gravity; and, furthermore, you learned from me that if, instead 
of a small ball I took a larger one, like that [changing one of the 
balls for a much larger one], there was much more of this attraction 
exerted; or, if I made this ball larger and larger, until, if it were 
possible, it became as large as the Earth itself — or I might take the 
Earth itself as the large ball — that then the attraction would become 
so powerful as to cause them to rush together in this manner [drop- 
ping the ivory ball]. You sit there upright, and I stand upright here, 
because we keep our centres of gravity properly balanced with re- 
spect to the earth; and I need not tell you that on the other side of 
this world the people are standing and moving about with their feet 
toward our feet, in a reversed position as compared with us, and all 
by means of this power of gravitation to the centre of the earth. 

I must not, however, leave the subject of gravitation without 
telling you something about its laws and regularity; and, first, as 
regards its power with respect to the distance that bodies are apart. 
If I take one of these balls and place it within an inch of the other, 



GRAVITY 23 

they attract each other with a certain power. If I hold it at a greater 
distance oflf, they attract with less power; and if I hold it at a greater 
distance still, their attraction is still less. Now this fact is of the 
greatest consequence; for, knowing this law, philosophers have dis- 
covered most wonderful things. You know that there is a planet, 
Uranus, revolving round the sun with us, but eighteen hundred 
millions of miles off, and because there is another planet as far off 
as three thousand millions of miles, this law of attraction, or gravi- 
tation, still holds good, and philosophers actually discovered this 
latter planet, Neptune, by reason of the effects of its attraction at 
this overwhelming distance. Now I want you clearly to understand 
what this law is. They say (and they are right) that two bodies 
attract each other inversely as the square of the distance — a sad 
jumble of words until you understand them; but I think we shall 
soon comprehend what this law is, and what is the meaning of the 
"inverse square of the distance." 

I have here (Fig. ii) a lamp. A, shining most intensely upon this 
disc, B, C, D, and this light acts as a sun by which I can get a 




Fig. II 



shadow from this little screen B F (merely a square piece of card), 
which, as you know, when I place it close to the large screen, just 
shadows as much of it as is exactly equal to its own size; but now 
let me take this card, E, which is equal to the other one in size, and 
place it midway between the lamp and the screen; now look at the 



24 FARADAY 

size of the shadow B D — it is four times the original size. Here, 
then, comes the "inverse square of the distance." This distance, A E, 
is one, and that distance, A B, is two, but that size E being one, this 
size B D of shadow is jour instead of two, which is the square of the 
distance, and, if I put the screen at one-third of the distance from 
the lamp, the shadow on the large screen would be nine times the 
size. Again, if I hold this screen here, at B F, a certain amount of 
light falls on it; and if I hold it nearer the lamp at E, more light 
shines upon it. And you see at once how much — exactly the quantity 
which I have shut off from the part of this screen, B D, now in 
shadow; moreover, you see that if I put a single screen here, at G, 
by the side of the shadow, it can only receive one-fourth of the pro- 
portion of light which is obstructed. That, then, is what is meant by 
the inverse of the square of the distance. This screen E is the bright- 
est because it is the nearest, and there is the whole secret of this 
curious expression, inversely as the square of the distance. Now if 
you can not perfectly recollect this when you go home, get a candle 
and throw a shadow of something — your profile, if you like — on the 
wall and then recede or advance, and you will find that your shadow 
is exactly in proportion to the square of the distance you are off the 
wall; and then, if you consider how much light shines on you at one 
distance, and how much at another, you get the inverse accordingly. 
So it is as regards the attraction of these two balls; they attract 
according to the square of the distance, inversely. I want you to try 
and remember these words, and then you will be able to go into all 
the calculations of astronomers as to the planets and other bodies, 
and tell why they move so fast, and why they go round the sun 
without falling into it and be prepared to enter upon many other 
interesting inquiries of the like nature. 

Let us now leave this subject which I have written upon the board 
under the word Force — Gravitation — and go a step farther. All 
bodies attract each other at sensible distances. I showed you the 
electric attraction on the last occasion (though I did not call it so) ; 
that attracts at a distance; and in order to make our progress a little 
more gradual, suppose I take a few iron particles [dropping some 
small fragments of iron on the table]. There! I have already told 
you that in all cases where bodies fall it is the particles that are 



COHESION 25 

attracted. You may consider these, then, as separate particles magni- 
fied, so as to be evident to your sight; they are loose from each other 
— they all gravitate — they all fall to the earth — for the force of gravi- 
tation never fails. Now I have here a centre of power which I will 
not name at present, and when these particles are placed upon it, 
see what an attraction they have for each other. 

Here I have an arch of iron filings (Fig. 12) regularly built up like 
an iron bridge, because I have put them within a sphere of action 
which will cause them to attract each other. See! I could let a mouse 
run through it; and yet, if I try to do the same thing with them 
here [on the table], they do not attract each other at all. It is that 
[the magnet] which makes them hold 
together. Now just as these iron particles 
hold together in the form of an elliptical 
bridge, so do the different particles of 
iron which constitute this nail hold to- 
gether and make it one. And here is a 
bar of iron; why, it is only because the 
different parts of this iron are so wrought 
as to keep close together by the attraction 
between the particles that it is held to- 
gether in one mass. It is kept together, in 
fact, merely by the attraction of one 
particle to another, and that is the point I want now to illustrate. 
If I take a piece of flint, and strike it with a hammer, and break it 
thus [breaking off a piece of the flint], I have done nothing more 
than separate the particles which compose these two pieces so far 
apart that their attraction is too weak to cause them to hold together, 
and it is only for that reason that there are now two pieces in the 
place of one. I will show you an experiment to prove that this 
attraction does still exist in those particles; for here is a piece of glass 
(for what was true of the flint and the bar of iron is true of the 
piece of glass, and is true of every other solid — they are all held 
together in the lump by the attraction between their parts), and I 
can show you the attraction between its separate particles; for if I 
take these portions of glass which I have reduced to very fine powder, 
you see that I can actually build them up into a solid wall by pressure 




Fig. 12 



2.6 FARADAY 

between two flat surfaces. The power which I thus have of build- 
ing up this wall is due to the attraction of the particles, forming, as 
it were, the cement which holds them together; and so in this case, 
where I have taken no very great pains to bring the particles together, 
you see perhaps a couple of ounces of finely pounded glass standing 
as an upright wall: is not this attraction most wonderful? That bar 
of iron one inch square has such power of attraction in its particles — 
giving to it such strength — that it will hold up twenty tons' weight 
before the little set of particles in the small space equal to one 
division across which it can be pulled apart will separate. In this 
manner suspension bridges and chains are held together by the 
attraction of their particles, and I am going to make an experiment 
which will show how strong is this attraction of the particles. [The 
lecturer here placed his foot on a loop of wire fastened to a support 
above, and swung with his whole weight resting upon it for some 
moments.] You see, while hanging here, all my weight is supported 
by these little particles of the wire, just as in pantomimes they some- 
times suspend gentlemen and damsels. 

How can we make this attraction of the particles a little more 
simple? There are many things which, if brought together properly, 
will show this attraction. Here is a boy's experiment (and I like a 
boy's experiment). Get a tobacco-pipe, fill it with lead, melt it, and 
then pour it out upon a stone, and thus get a clean piece of lead 
(this is a better plan than scraping it; scraping alters the condition 
of the surface of the lead) . I have here some pieces of lead which I 
melted this morning for the sake of making them clean. Now these 
pieces of lead hang together by the attraction of their particles, and 
if I press these two separate pieces close together, so as to bring their 
particles within the sphere of attraction, you will see how soon they 
become one. I have merely to give them a good squeeze, and draw 
the upper piece slightly round at the same time, and here they are 
as one, and all the bending and twisting I can give them will not 
separate them again; I have joined the lead together, not with solder, 
but simply by means of the attraction of the particles. 

This, however, is not the best way of bringing those particles 
together; we have many better plans than that; and I will show you 
one that will do very well for juvenile experiments. There is some 



COHESION 27 

alum crystallized very beautifully by nature (for all things are far 
more beautiful in their natural than their artificial form), and here 
I have some of the same alum broken into fine powder. In it I have 
destroyed that force of which I have placed the name on this board 
— Cohesion, or the attraction exerted between the particles of bodies 
to hold them together. Now I am going to show you that if we take 
this powdered alum and some hot water, and mix them together, I 
shall dissolve the alum; all the particles will be separated by the 
water far more completely than they are here in the powder; but 
then, being in the water, they will have the opportunity as it cools 
(for that is the condition which favors their coalescence) of uniting 
together again and forming one mass (^) . 

Now, having brought the alum into solution, I will pour it into 
this glass basin, and you will, to-morrow, find that these particles of 
alum which I have put into the water, and so separated that they are 
no longer solid, will, as the water cools, come together and cohere, 
and by to-morrow morning we shall have a great deal of the alum 
crystallized out — that is to say, come back to the solid form. [The 
lecturer here poured a little of the hot solution of alum into the glass 
dish, and when the latter had thus been made warm, the remainder 
of the solution was added.] I am now doing that which I advise 
you to do if you use a glass vessel, namely, warming it slowly and 
gradually; and in repeating this experiment, do as I do — pour the 
liquid out gently, leaving all the dirt behind in the basin; and 
remember that the more carefully and quietly you make this experi- 
ment at home, the better the crystals. To-morrow you will see the 
particles of alum drawn together; and if I put two pieces of coke in 
some part of the solution (the coke ought first to be washed very 
clean, and dried), you will find to-morrow that we shall have a 
beautiful crystallization over the coke, making it exactly resemble a 
natural mineral. 

Now how curiously our ideas expand by watching these conditions 
of the attraction of cohesion! how many new phenomena it gives 

* Crystallization of alum. The solution must be saturated — that is, it must contain 
as much alum as can possibly be dissolved. In making the solution, it is best to add 
powdered alum to hot water as long as it dissolves; and when no more is taken up, 
allow the solution to stand a few minutes, and then pour it off from the dirt and 
undissolved alum. 



28 FARADAY 

us beyond those of the attraction of gravitation! See how it gives us 
great strength. The things we deal with in building up the struc- 
tures on the earth are of strength — we use iron, stone, and other 
things of great strength; and only think that all those structures you 
have about you — think of the Great Eastern, if you please, which is 
of such size and power as to be almost more than man can manage — 
are the result of this power of cohesion and attraction. 

I have here a body in which I believe you will see a change taking 
place in its condition of cohesion at the moment it is made. It is at 
first yellow; it then becomes a fine crimson red. Just watch when I 
pour these two liquids together — both colorless as water. [The 
lecturer here mixed together solutions of perchloride of mercury 
and iodide of potassium, when a yellow precipitate of biniodide of 
mercury fell down, which almost immediately became crimson red.] 
Now there is a substance which is very beautiful, but see how it is 
changing color. It was reddish-yellow at first, but it has now become 
red ('). I have previously prepared a little of this red substance, 
which you see formed in the liquid, and have put some of it upon 
paper [exhibiting several sheets of paper coated with scarlet biniodide 
of mercury (*)]. There it is — the same substance spread upon paper; 
and there, too, is the same substance; and here is some more of it 
[exhibiting a piece of paper as large as the other sheets, but having 
only very little red color on it, the greater part being yellow] — a 
little more of it, you will say. Do not be mistaken; there is as much 
upon the surface of one of these pieces of paper as upon the other. 
What you see yellow is the same thing as the red body, only the 
attraction of cohesion is in a certain degree changed, for I will take 
this red body, and apply heat to it (you may perhaps see a little 

''Red precipitate of biniodide of mercury. A little care is necessary to obtain this 
precipitate. The solution of iodide of potassium should be added to the solution of 
perchloride of mercury (corrosive sublimate) very gradually. The red precipitate 
which first falls is redissolved when the liquid is stirred: when a litrie more of the 
iodide of potassium is added a pale red precipitate is formed, which, on the farther 
addition of the iodide, changes into the brilliant scarlet biniodide of mercury. If 
too much iodide of potassium is added, the scarlet precipitate disappears, and a 
colorless solution is left. 

^ Paper coated with scarlet biniodide of mercury. In order to fix the biniodide on 
paper, it must be mixed with a litde weak gum water, and then spread over the 
paper, which must be dried without heat. 

Biniodide of mercury is said to be dimorphous; that is, is able to assume two different 
forms. 



COHESION 29 

smoke arise, but that is of no consequence), and if you look at it it 
will first of all darken — but see how it is becoming yellow. I have 
now made it all yellow, and, what is more, it will remain so; but if I 
take any hard substance, and rub the yellow part with it, it will 
immediately go back again to the red condition [exhibiting the ex- 
periment]. There it is. You see the red is not put bac\, but brought 
bac}{ by the change in the substance. Now [warming it over the 
spirit lamp] here it is becoming yellow again, and that is all because 
its attraction of cohesion is changed. And what will you say to me 
when I tell you that this piece of common charcoal is just the same 
thing, only differently coalesced, as the diamonds which you wear.? 
(I have put a specimen outside of a piece of straw which was 
charred in a particular way — ^it is just like black lead.) Now this 
charred straw, this charcoal, and these diamonds, are all of them the 
same substance, changed but in their properties as respects the force 
of cohesion. 

Here is a piece of glass [producing a piece of plate-glass about two 
inches square]. (I shall want this afterward to look to and examine 





Fig. 13 



Fig. 14 



its internal condition), and here is some of the same sort of glass 

differing only in its power of cohesion, because while yet melted it 

had been dropped into cold water [exhibiting a "Prince Rupert's 

drop,"(°) (Fig. 13)], and if I take one of these little tear-like pieces 

and break off ever so little from the point, the whole will at once 

burst and fall to pieces. I will now break off a piece of this. [The 

^"Prince Rupert's Drops." These are made by pouring drops of melted green 
glass into cold water. They were not, as is commonly supposed, invented by Prince 
Rupert, but were first brought to England by him in 1660. They excited a great 
deal o£ curiosity, and were considered "a kind of miracle in nature." 



30 FARADAY 

lecturer nipped off a small piece from the end of one of Rupert's 
drops, whereupon the whole immediately fell to pieces.] There! 
you see the solid glass has suddenly become powder, and more than 
that, it has knocked a hole in the glass vessel in which it was held. 
I can show the effect better in this bottle of water, and it is very 
likely the whole bottle will go. [A 6-oz. vial was filled with water, 
and a Rupert's drop placed in it with the point of the tail just pro- 
jecting out; upon breaking the tip off, the drop burst, and the shock, 
being transmitted through the water to the sides of the bottle, 
shattered the latter to pieces.] 

Here is another form of the same kind of experiment. I have 
here some more glass which has not been annealed [showing some 
thick glass vessels('°) (Fig. 14)], and if I take one of these glass 
vessels and drop a piece of pounded glass into it (or I will take some 
of these small pieces of rock crystal; they have the advantage of 
being harder than glass), and so make the least scratch upon the 
inside, the whole bottle will break to pieces — it can not hold together. 
[The lecturer here dropped a small fragment of rock crystal into one 
of these glass vessels, when the bottom immediately came out and 
fell upon the plate.] There!, it goes through, just as it would through 
a sieve. 

Now I have shown you these things for the purpose of bringing 
your minds to see that bodies are not merely held together by this 
power of cohesion, but that they are held together in very curious 
ways. And suppose I take some things that are held together by this 
force, and examine them more minutely. I will first take a bit of 
glass, and if I give it a blow with a hammer I shall just break it to 
pieces. You saw how it was in the case of the flint when I broke 
the piece off; a piece of a similar kind would come off, just as you 
would expect; and if I were to break it up still more, it would be, 
as you have seen, simply a collection of small particles of no definite 
shape or form. But supposing I take some other thing— this stone, 
for instance (Fig. 15) [taking a piece of mica(")], and if I hammer 
this stone I may batter it a great deal before I can break it up. I may 
even bend it without breaking it — that is to say, I may bend it in 

" Thick, glass vessels. They are called Proofs or Bologna phials. 
" Mica. A silicate of alumina and magnesia. It has a bright metallic lustre; hence 
its name, from mico, to shine. 



COHESION 31 

one particular direction without breaking it much, although I feel in 
my hands that I am doing it some injury. But now, if I take it by 
the edges, I find that it breaks up into leaf after leaf in a most 
extraordinary manner. Why should it break up like that? Not 
because all stones do, or all crystals; for there is some salt (Fig. 16) — 
you know what common salt is ('^) ; here is a piece of this salt, which 
by natural circumstances has had its particles so brought together 




Fig. 15 Fig. 16 Fig. 17 

that they have been allowed free opportunity of combining or coa- 
lescing, and you shall see what happens if I take this piece of salt 
and break it. It does not break as flint did, or as the mica did, but 
with a clean sharp angle and exact surfaces, beautiful and glittering 
as diamonds [breaking it by gentle blows with a hammer]; there is 
a square prism which I may break up into a square cube. You see 
these fragments are all square; one side may be longer than the 
other, but they will only split up so as to form square or oblong pieces 
with cubical sides. Now I go a little farther, and I find another 
stone (Fig. 17) [Iceland or calc-spar] (") which I may break in a 
similar way, but not with the same result. Here is a piece which I 
have broken off, and you see there are plain surfaces perfectly 
regular with respect to each other, but it is not cubical — it is what 
we call a rhomboid. It still breaks in three directions most beauti- 
fully and regularly with polished surfaces, but with sloping sides, 
not like the salt. Why not.? It is very manifest that this is owing 
to the attraction of the particles one for the other being less in the 
direction in which they give way than in other directions. I have 
on the table before me a number of little bits of calcareous spar, and 

'^ Common salt or chloride of sodium crystallizes in the form of solid cubes, which, 
aggregated together, form a mass, which may be broken up into the separate cubes. 
'' Iceland or calc-spar. Native carbonate of lime in its primitive crystalline form. 



32 FARADAY 

I recommend each of you to take a piece home, and then you can 
take a knife and try to divide it in the direction of any of the 
surfaces already existing. You will be able to do it at once; but 
if you try to cut it across the crystals, you can not; by hammering you 
may bruise and break it up, but you can only divide it into these 
beautiful little rhomboids. 

Now I want you to understand a little more how this is, and for 
this purpose I am going to use the electric Hght again. You see we 
can not look into the middle of a body like this piece of glass. We 
perceive the outside form and the inside form, and we look through 
it, but we can not well find out how these forms become so, and I 




Fig. 1 8 

want you, therefore, to take a lesson in the way in which we use a 
ray of light for the purpose of seeing what is in the interior of bodies. 
Light is a thing which is, so to say, attracted by every substance that 
gravitates (and we do not know any thing that does not). AH 
matter affects light more or less by what we may consider as a kind 
of attraction, and I have arranged (Fig. i8) a very simple experiment 
upon the floor of the room for the purpose of illustrating this. I 
have put into that basin a few things which those who are in the 
body of the theatre will not be able to see, and I am going to make 
use of this power which matter possesses of attracting a ray of light. 
If Mr. Anderson pours some water, gently and steadily, into the 
basin, the water will attract the rays of light downward, and the 
piece of silver and the sealing-wax will appear to rise up into 
the sight of those who were before not high enough to see over the 
side of the basin to its bottom. [Mr. Anderson here poured water 
into the basin, and upon the lecturer asking whether any body could 



COHESION 33 

see the silver and sealing-wax, he was answered by a general affirm- 
ative.] Now I suppose that every body can see that they are not at all 
disturbed, while from the way they appear to have risen up you 
would imagine the bottom of the basin and the articles in it were 
two inches thick, although they are only one of our small silver 
dishes and a piece of sealing-wax which I have put there. The light 
which now goes to you from that piece of silver was obstructed by 
the edge of the basin when there was no water there, and you were 
unable to see anything of it; but when we poured in water the rays 
were attracted down by it over the edge of the basin, and you were 
thus enabled to see the articles at the bottom. 

I have shown you this experiment first, so that you might under- 
stand how glass attracts light, and might then see how other sub- 
stances like rock-salt and calcareous spar, mica, and other stones, 
would affect the light; and, if Dr. Tyndall will be good enough to 
let us use his light again, we will first of all show you how it may 
be bent by a piece of glass (Fig. 19). [The electric lamp was again 




Fig. 19 

lit, and the beam of parallel rays of light which it emitted was bent 
about and decomposed by means of the prism.] Now, here you see, 
if I send the light through this piece of plain glass, A, it goes straight 
through without being bent (unless the glass be held obliquely, and 
then the phenomenon becomes more complicated) ; but if I take this 
piece of glass, B [a prism], you see it will show a very different 
effect. It no longer goes to that wall, but it is bent to this screen, C, 
and how much more beautiful it is now [throwing the prismatic 
spectrum on the screen]. This ray of light is bent out of its course 
by the attraction of the glass upon it; and you see I can turn and 
twist the rays to and fro in different parts of the room, just as I 
please. Now it goes there, now here. [The lecturer projected the 



34 FARADAY 

prismatic spectxum about the theatre.] Here I have the rays once 
more bent on to the screen, and you see how wonderfully and beauti- 
fully that piece of glass not only bends the light by virtue of its 
attraction, but actually splits it up into different colors. Now I 
want you to understand that this piece of glass [the prism], being 
perfectly uniform in its internal structure, tells us about the action 
of these other bodies which are not uniform — which do not merely 
cohere, but also have within them, in different parts, different de- 
grees of cohesion, and thus attract and bend the light with varying 
powers. We will now let the light pass through one or two of these 
things which I just now showed you broke so curiously; and, first 
of all, I will take a piece of mica. Here, you see, is our ray of light : 
we have first to make it what we call polarized; but about that you 
need not trouble yourselves; it is only to make our illustration more 
clear. Here, then, we have our polarized ray of light, and I can so 
adjust it as to make the screen upon which it is shining either light 
or dark, although I have nothing in the course of this ray of light 
but what is perfectly transparent [turning the analyzer round]. I 
will now make it so that it is quite dark, and we will, in the first 
instance, put a piece of common glass into the polarized ray so as to 
show you that it does not enable the light to get through. You see 
the screen remains dark. The glass, then, internally, has no effect 
upon light. [The glass was removed and a piece of mica introduced.] 
Now there is the mica which we split up so curiously into leaf after 
leaf, and see how that enables the light to pass through to the screen, 
and how, as Dr. Tyndall turns it round in his hand, you have those 
different colors, pink, and purple, and green, coming and going 
most beautifully; not that the mica is more transparent than the 
glass, but because of the different manner in which its particles are 
arranged by the force of cohesion. 

Now we will see how calcareous spar acts upon this light — that 
stone which split up into rhombs, and of which you are each of you 
going to take a little piece home. [The mica was removed, and a 
piece of calc-spar introduced at A.] See how that turns the light 
round and round, and produces these rings and that black cross 
(Fig. 20). Look at those colors: are they not most beautiful for you 
and for me? (for I enjoy these things as much as you do). In what 



COHESION 35 

a wonderful manner they open out to us the internal arrangement of 
the particles of this calcareous spar by the force of cohesion. 

And now I will show you another experiment. Here is that piece 
of glass which before had no action upon the light. You shall see 
what it will do when we apply pressure to it. Here, then, we have 
our ray of polarized light, and I will first of all show you that the 






f 



Fig. 20 



glass has no effect upon it in its ordinary state; when I place it in 
the course of the light, the screen still remains dark. Now Dr. 
Tyndall will press that bit of glass between three little points, one 
point against two, so as to bring a strain upon the parts, and you 
will see what a curious effect that has. [Upon the screen two white 
dots gradually appeared.] Ah! these points show the position of 
the strain; in these parts the force of cohesion is being exerted in a 
different degree to what it is in the other parts, and hence it allows 
the light to pass through. How beautiful that is! how it malces the 
light come through some parts and leaves it dark in others, and all 
because we weaken the force of cohesion between particle and 
particle. Whether you have this mechanical power of straining, or 
whether we take other means, we get the same result; and, indeed, 
I will show you by another experiment that if we heat the glass in 
one part, it will alter its internal structure and produce a similar 
effect. Here is a piece of common glass, and if I insert this in the 
path of the polarized ray, I believe it will do nothing. There is the 
common glass [introducing it]. No light passes through; the screen 
remains quite dark; but I am going to warm this glass in the lamp, 
and you know yourselves that when you pour warm water upon 



36 FARADAY 

glass you put a strain upon it sufficient to break it sometimes — some- 
thing like there was in the case of the Prince Rupert's drops. [The 
glass was warmed in the spirit lamp, and again placed across the 
ray of light.] Now you see how beautifully the light goes through 
those parts which are hot, making dark and light lines just as the 
crystal did, and all because of the alteration I have effected in its 
internal condition; for these dark and light parts are a proof of the 
presence of forces acting and dragging in different directions within 
the solid mass. 



LECTURE III 

COHESION— CHEMICAL AFFINITY 

WE will first return for a few minutes to one of the experi- 
ments made yesterday. You remember what we put 
together on that occasion — ^powdered alum and warm 
water. Here is one of the basins then used. Nothing has been done 
to it since; but you will find, on examining it, that it no longer 
contains any powder, but a number of beautiful crystals. Here also 
are the pieces of coke which I put into the other basin; they have a 
fine mass of crystals about them. That other basin I will leave as 
it is. I will not pour the water from it, because it will show you that 
the particles of alum have done something more than merely crystal- 
lize together. They have pushed the dirty matter from them, laying 
it around the outside or outer edge of the lower crystals — squeezed 
out, as it were, by the strong attraction which the particles of alum 
have for each other. 

And now for another experiment. We have already gained a 
knowledge of the manner in which the particles of bodies — of solid 
bodies — attract each other, and we have learned that it makes cal- 
careous spar, and so forth, crystallize in these regular forms. Now 
let me gradually lead your minds to a knowledge of the means we 
possess of making this attraction alter a little in its force; either of 
increasing, or diminishing, or, apparently, of destroying it alto- 
gether. I will take this piece of iron [a rod of iron about two feet 
long and a quarter of an inch in diameter]. It has at present a great 
deal of strength, due to its attraction of cohesion; but if Mr. Ander- 
son will make part of this red-hot in the fire, we shall then find that 
it will become soft, just as sealing-wax will when heated, and we 
shall also find that the more it is heated the softer it becomes. Ah! 
but what does soft mean? Why, that the attraction between the 
particles is so weakened that it is no longer sufficient to resist the 
power we bring to bear upon it. [Mr. Anderson handed to the 

37 



38 FARADAY 

lecturer the iron rod, with one end red-hot, which he showed could 
be easily twisted about with a pair of pliers.] You see I now find no 
difhculty in bending this end about as I like, whereas I can not bend 
the cold part at all. And you know how the smith takes a piece of 
iron and heats it in order to render it soft for his purpose: he acts 
upon our principle of lessening the adhesion of the particles, 
although he is not exactly acquainted with the terms by which we 
express it. 

And now we have another point to examine, and this water is 
again a very good substance to take as an illustration (as philoso- 
phers we call it all water, even though it be in the form of ice or 
steam). Why is this water hard? [pointing to a block of ice]; 
because the attraction of the particles to each other is sufficient to 
make them retain their places in opposition to force applied to it. 
But what happens when we make the ice warm? Why, in that 
case we diminish to such a large extent the power of attraction that 
the solid substance is destroyed altogether. Let me illustrate this: I 
will take a red-hot ball of iron [Mr. Anderson, by means of a pair 
of tongs, handed to the lecturer a red-hot ball of iron, about two 
inches in diameter], because it will serve as a convenient source of 
heat [placing the red-hot iron in the centre of the block of ice]. You 
see I am now melting the ice where the iron touches it. You see the 
iron sinking into it; and while part of the solid water is becoming 
liquid, the heat of the ball is rapidly going off. A certain part of 
the water is actually rising in steam, the attraction of some of the 
particles is so much diminished that they can not even hold together 
in the liquid form, but escape as vapor. At the same time, you see 
I can not melt all this ice by the heat contained in this ball. In 
the course of a very short time I shall find it will have become quite 
cold. 

Here is the water which we have produced by destroying some of 
the attraction which existed between the particles of the ice, for 
below a certain temperature the particles of water increase in their 
mutual attraction and become ice; and above a certain temperature 
the attraction decreases and the water becomes steam. And exactly 
the same thing happens with platinum, and nearly every substance 
in nature; if the temperature is increased to a certain point it becomes 



COHESION 39 

liquid and a farther increase converts it into a gas. Is it not a glorious 
thing for us to look at the sea, the rivers, and so forth, and to know 
that this same body in the northern regions is all solid ice and ice- 
bergs, while here, in a warmer climate, it has its attraction of 
cohesion so much diminished as to be liquid water ? Well, in dimin- 
ishing this force of attraction between the particles of ice, we made 
use of another force, namely, that of heat; and I want you now to 
understand that this force of heat is always concerned when water 
passes from the solid to the liquid state. If I melt ice in other ways 
I can not do without heat (for we have the means of making ice 
liquid without heat — that is to say, without using heat as a direct 
cause) . Suppose, for illustration, I make a vessel out of this piece of 
tinfoil [bending the foil up into the shape of a dish]. I am making 
it metallic, because I want the heat which I am about to deal with to 
pass readily through it; and I am going to pour a little water on 
this board, and then place the tin vessel on it. Now if I put some of 
this ice into the metal dish, and then proceed to make it liquid by 
any of the various means we have at our command, it still must 
take the necessary quantity of heat from something, and in this case 
it will take the heat from the tray, and from the water underneath, 
and from the other things round about. Well, a little salt added to 
the ice has the power of causing it to melt, and we shall very shortly 
see the mixture become quite fluid, and you will then find that the 
water beneath will be frozen — frozen because it has been forced to 
give up that heat which is necessary to keep it in the liquid state 
to the ice on becoming liquid. I remember once, when I was a boy, 
hearing of a trick in a country ale-house: the point was how to 
melt ice in a quart pot by the fire and freeze it to the stool. Well, 
the way they did it was this : they put some pounded ice in a pewter 
pot, and added some salt to it, and the consequence was that when 
the salt was mixed with it, the ice in the pot melted (they did not 
tell me any thing about the salt and they set the pot by the lire, just 
to make the result more mysterious), and in a short time the pot and 
the stool were frozen together, as we shall very shortly find it to be 
the case here, and all because salt has the power of lessening the 
attraction between the particles of ice. Here you see the tin dish is 
frozen to the board; I can even lift the little stool up by it. 



40 FARADAY 

This experiment can not, I think, fail to impress upon your minds 
the fact that whenever a soHd body loses some of that force of attrac- 
tion by means of which it remains solid, heat is absorbed; and if, 
on the other hand, we convert a liquid into a solid, e. g., water into 
ice, a corresponding amount of heat is given out. I have an experi- 
ment showing this to be the case. Here (Fig. 21) is a bulb, A, filled 




Fig. 21 

with air, the tube from which dips into some colored liquid in the 
vessel B. And I dare say you know that if I put my hand on the 
bulb A, and warm it, the colored liquid which is now standing in 
the tube at C will travel forward. Now we have discovered a means, 
by great care and research into the properties of various bodies, of 
preparing a solution of a salt (") which, if shaken or disturbed, will 
at once become a solid; and as I explained to you just now (for what 
is true of water is true of every other liquid), by reason of its 
becoming solid heat is evolved, and I can make this evident to you 
by pouring it over this bulb; there! it is becoming solid; and look 
at the colored liquid, how it is being driven down the tube, and 
how it is bubbling out through the water at the end; and so we learn 
this beautiful law of our philosophy, that whenever we diminish 
the attraction of cohesion we absorb heat, and whenever we increase 
that attraction heat is evolved. This, then, is a great step in advance, 
for you have learned a great deal in addition to the mere circum- 
stance that particles attract each other. But you must not now 
suppose that because they are liquid they have lost their attraction 

^* Solution of a salt. Acetate of soda. A solution saturated, or nearly so, at the 
boiling point, is necessary, and it must be allowed to cool, and remain at rest until 
the experiment is made. 



COHESION 41 

of cohesion; for here is the fluid mercury, and if I pour it from one 
vessel into another, I find that it will form a stream from the bottle 
down to the glass — a continuous rod of fluid mercury, the particles 
of which have attraction sufficient to make them hold together all 
the way through the air down to the glass itself; and if I pour water 
quietly from a jug, I can cause it to run in a continuous stream in 
the same manner. Again: let me put a little water on this piece 
of plate glass, and then take another plate of glass and put it on the 
water; there! the upper plate is quite free to move, gliding about 
on the lower one from side to side; and yet, if I take hold of the 
upper plate and lift it up straight, the cohesion is so great that the 
lower one is held up by it. See how it runs about as I move the 
upper one, and this is all owing to the strong attraction of the 
particles of the water. Let me show you another experiment. If I 
take a little soap and water — not that the soap makes the particles 
of the water more adhesive one for the other, but it certainly has 
the power of continuing in a better manner the attraction of the 
particles (and let me advise you, when about to experiment with 
soap-bubbles, to take care to have every thing clean and soapy). I 
will now blow a bubble, and that I may be able to talk and blow a 
bubble too, I will take a plate with a little of the soapsuds in it, and 
will just soap the edges of the pipe and blow a bubble on to the 
plate. Now there is our bubble. Why does it hold together in this 
manner? Why, because the water of which it is composed has an 
attraction of particle for particle — so great, indeed, that it gives to 
this bubble the very power of an India-rubber ball; for you see, if I 
introduce one end of this glass tube into the bubble, that it has the 
power of contracting so powerfully as to force enough air through 
the tube to blow out a light (Fig. 22) ; the light is blown out. And 
look! see how the bubble is disappearing — see how it is getting 
smaller and smaller. 

There are twenty other experiments I might show you to illustrate 
this power of cohesion of the particles of liquids. For instance, what 
would you propose to me if, having lost the stopper out of this 
alcohol bottle, I should want to close it speedily with something near 
at hand. Well, a bit of paper would not do, but a piece of linen cloth 
would, or some of this cotton wool which I have here. I will put a 



42 FARADAY 

tuft of it into the neck of the alcohol bottle, and you see, when I 
turn it upside down, that it is perfectly well stoppered so far as the 
alcohol is concerned; the air can pass through, but the alcohol can 
not. And if I were to take an oil vessel this plan would do equally 
well, for in former times they used to send us oil from Italy in 
flasks stoppered only with cotton wool (at the present time the cotton 
is put in after the oil has arrived here, but formerly it used to be 
sent so stoppered). Now if it were not for the particles of liquid 
cohering together, this alcohol would run out; and if I had time I 




Fig. 22 



Fig. 23 



could have shown you a vessel with the top, bottom, and sides 
altogether formed like a sieve, and yet it would hold water, owing 
to the cohesion. 

You have now seen that the solid water can become fluid by the 
addition of heat, owing to this lessening the attractive force between 
its particles, and yet you see that there is a good deal of attractive 
force remaining behind. I want now to take you another step 
beyond. We saw that if we continued applying heat to the water 
(as indeed happened with our piece of ice here), that we did at last 
break up that attraction which holds the liquid together, and I am 
about to take some other (any other liquid would do, but ether 
makes a better experiment for my purpose) in order to illustrate 
what will happen when this cohesion is broken up. Now this liquid 
ether, if exposed to a very low temperature, will become a solid; but 
if we apply heat to it, it becomes vapor; and I want to show you the 
enormous bulk of the substance in this new form: when we make 
ice into water, we lessen its bulk; but when we convert water into 
steam, we increase it to an enormous extent. You see it is very clear 
that as I apply heat to the liquid I diminish its attraction of co- 



COHESION 43 

hesion; it is now boiling, and I will set fire to the vapor, so that you 
may be enabled to judge of the space occupied by the ether in this 
form by the size of its flame; and you now see what an enormously 
bulky flame I get from that small volume of ether below. The heat 
from the spirit lamp is now being consumed, not in making the 
ether any warmer, but in converting it into vapor; and if I desired 
to catch this vapor and condense it (as I could without much diffi- 
culty), I should have to do the same as if I wished to convert steam 
into water and water into ice: in either case it would be necessary to 
increase the attraction of the particles by cold or otherwise. So largely 
is the bulk occupied by the particles increased by giving them this 
diminished attraction, that if I were to take a portion of water 
a cubic inch in bulk (A, Fig. 23), I should produce a volume of 
steam of that size, B [1,700 cubic inches; nearly a cubic foot], so 
greatly is the attraction of cohesion diminished by heat; and yet it 
still remains water. You can easily imagine the consequences which 
are due to this change in volume by heat — the mighty powers of 
steam and the tremendous explosions which are sometimes pro- 
duced by this force of water. I want you now to see another experi- 
ment, which will perhaps give you a better illustration of the bulk 
occupied by a body when in the state of vapor. Here is a substance 
which we call iodine, and I am about to submit this solid body to 
the same kind of condition as regards heat that I did the water and 
the ether [putting a few grains of iodine into a hot glass globe, 
which immediately became filled with the violet vapor], and you 
see the same kind of change produced. Moreover, it gives us the 
opportunity of observing how beautiful is the violet-colored vapor 
from this black substance, or rather the mixture of the vapor with 
air (for I would not wish you to understand that this globe is 
entirely filled with the vapor of iodine) . 

If I had taken mercury and converted it into vapor (as I could 
easily do), I should have a perfectly colorless vapor; for you must 
understand this about vapors, that bodies in what we call the vapor- 
ous or the gaseous state are always perfectly transparent, never 
cloudy or smoky; they are, however, often colored, and we can 
frequently have colored vapors or gases produced by colorless par- 
ticles themselves mixing together, as in this case [the lecturer here 



44 FARADAY 

inverted a glass cylinder full of binoxide of nitrogen (") over a 
cylinder of oxygen, when the dark red vapor of hyponitrous acid 
was produced]. Here also you see a very excellent illustration of the 
efifect of a power of nature which we have not as yet come to, but 
which stands next on our list — Chemical Affinity. And thus you 
see we can have a violet vapor or an orange vapor, and different 
other kinds of vapor, but they are always perfectly transparent, or 
else they would cease to be vapors. 

I am now going to lead you a step beyond this consideration of 
the attraction of the particles for each other. You see we have come 
to understand that, if we take water as an illustration, whether it be 
ice, or water, or steam, it is always to be considered by us as water. 
Well, now prepare your minds to go a little deeper into the subject. 
We have means of searching into the constitution of water beyond 
any that are afforded us by the action of heat, and among these one 
of the most important is that force which we call voltaic electricity, 
which we used at our last meeting for the purpose of obtaining 
light, and which we carried about the room by means of these 
wires. This force is produced by the battery behind me, to which, 
however, I will not now refer more particularly; before we have 
done we shall know more about this battery, but it must grow up 
in our knowledge as we proceed. Now here (Fig. 24) is a portion of 
water in this little vessel, C, and besides the water there are two 
plates of the metal platinum, which are connected with the wires 
(A and B) coming outside, and I want to examine that water, and 
the state and the condition in which its particles are arranged. If I 
were to apply heat to it you know what we should get; it would 
assume the state of vapor, but it would nevertheless remain water, 
and would return to the liquid state as soon as the heat was removed. 
Now by means of these wires (which are connected with the battery 
behind me, and come under the floor and up through the table) we 
shall have a certain amount of this new power at our disposal. Here 

'^ Binoxide of nitrogen and hyponitrous acid. Binoxide of nitrogen is formed when 
nitric acid and a little water are added to some copper turnings. It produces deep 
red fumes as soon as it comes in contact with the air, by combining with the oxygen 
of the latter to form hyponitrous acid. Binoxide of nitrogen is composed of two parts 
of oxygen and one part of nitrogen; hyponitrous acid is composed of one part of 
nitrogen and three parts of oxygen. 



CHEMICAL AFFINITY 45 

you see it is [causing the ends of the wires to touch] — that is the 
electric Hght we used yesterday, and by means of these wires we 
can cause water to submit itself to this power; for the moment I put 
them into metallic connection (at A and B), you see the water boil- 
ing in that little vessel (C), and you hear the bubbling of the gas 
that is going through the tube (D). See how I am converting the 
water into vapor; and if I take a httle vessel (E), and fill it with. 
water, and put it into the trough over the end of the tube (D), 




Fig. 24 

there goes the vapor ascending into the vessel. And yet that is not 
steam, for you know that if steam is brought near cold water, if 
would at once condense, and return back again to water; this, then, 
can not be steam, for it is bubbling through the cold water in this 
trough; but it is a vaporous substance, and we must therefore 
examine it carefully, to see in what way the water has been changed. 
And now, in order to give you a proof that it is not steam, I am 
going to show you that it is combustible; for if I take this small 
vessel to a light, the vapor inside explodes in a manner that steam 
could never do. 

I will now fill this large bell-jar (F) with water; and I propose 
letting the gas ascend into it, and I will then show you that we can 
reproduce the water back again from the vapor or air that is there. 
Here is a strong glass vessel (G), and into it we will let the gas 
(from F) pass. We will there fire it by the electric spark, and then, 



46 FARADAY 

after the explosion, you will find that we have got the water back 
again; it will not be much, however, for you will recollect that I 
showed you how small a portion of water produced a very large 
volume of vapor. Mr. Anderson will now pump all the air out of 
this vessel (G), and when I have screwed it on to the top of ouf 
jar of gas (F), you will see, upon opening the stop-cocks (H H H), 
the water will jump up, showing that some of the gas has passed 
into the glass vessel. I will now shut these stop-cocks, and we shall 
be able to send the electric spark through the gas by means of the 
wires (I, K) in the upper part of the vessel, and you will see it burn 
with a most intense flash. [Mr. Anderson here brought a Ley den 
jar, which he discharged through the confined gas by means of the 
wires I, K.] You saw the flash, and now that you may see that there 
is no longer any gas remaining, if I place it over the jar and open 
the stop-cocks again, up will go the gas, and we can have a second 
combustion; and so I might go on again and again, and I should 
continue to accumulate more and more of the water to which the 
gas has returned. Now is not this curious.'' In this vessel (C) we 
can go on making from water a large bulk of permanent gas, as we 
call it, and then we can reconvert it into water in this way. [Mr. 
Anderson brought in another Ley den jar, which, however, from 
some cause, would not ignite the gas. It was therefore recharged, 
when the explosion took place in the desired manner.] How beau- 
tifully we get our results when we are right in our proceedings! It 
is not that Nature is wrong when we make a mistake. Now I will 
lay this vessel (G) down by my right hand, and you can examine 
it by-and-by; there is not very much water flowing down, but there 
is quite sufficient for you to see. 

Another wonderful thing about this mode of changing the con- 
dition of the water is this : that we are able to get the separate parts 
of which it is composed at a distance the one from the other, and 
to examine them, and see what they are like, and how many of them 
there are; and for this purpose I have here some more water in a 
slightly different apparatus to the former one (Fig. 25), and if I place 
this in connection with the wires of the battery (at A, B), I shall get 
a similar decomposition of the water at the two platinum plates. 




Fig. 25 



CHEMICAL AFFINITY 47 

Now I will put this little tube (O) over there, and that will collect 
the gas together that comes from this side (A), and this tube (H) 
will collect the gas that comes from the other side (B), and I think 
we shall soon be able to see a difference. In this apparatus the wires 
are a good way apart from each other, 
and it now seems that each of them is 
capable of drawing off particles from the 
water and sending them off, and you see 
that one set of particles (H) is coming off 
twice as fast as those collected in the other 
tube (O). Something is coming out of 
the water there (at H) which burns [set- 
ting fire to the gas]; but what comes out 
of the water here (at O), although it will 
not burn, will support combustion very 
vigorously. [The lecturer here placed a match with a glowing tip in 
the gas, when it immediately rekindled.] 

Here, then, we have two things, neither of them being water 
alone, but which we get out of the water. Water is therefore com- 
posed of two substances different to itself, which appear at separate 
places when it is made to submit to the force which I have in these 
wires; and if I take an inverted tube of water and collect this gas 
(H), you will see that it is by no means the same as the one we 
collected in the former apparatus (Fig. 24). That exploded with a 
loud noise when it was lighted, but this will burn quite noiselessly : it 
is called hydrogen; and the other we call oxygen — that gas which so 
beautifully brightens up all combustion, but does not burn of itself. 
So now we see that water consists of two kinds of particles attracting 
each other in a very different manner to the attraction of gravitation 
or cohesion, and this new attraction we call chemical affinity, or the 
force of chemical action between different bodies; we are now no 
longer concerned with the attraction of iron for iron, water for water, 
wood for wood, or like bodies for each other, as we were when deal- 
ing with the force of cohesion; we are dealing with another kind 
of attraction — the attraction between particles of a different nature 
one to the other. Chemical affinity depends entirely upon the energy 



48 FARADAY 

with which particles of different kinds attract each other. Oxygen 
and hydrogen are particles of different kinds, and it is their attrac- 
tion to each other which makes them chemically combine and pro- 
duce water. 

I must now show you a little more at large what chemical affinity 
is. I can prepare these gases from other substances as well as from 
water; and we will now prepare some oxygen: here is another sub- 
stance which contains oxygen — chlorate of potash; I will put some of 
it into this glass retort, and Mr. Anderson will apply heat to it: 
we have here different jars filled with water, and when, by the 
application of heat, the chlorate of potash is decomposed, we will 
displace the water, and fill the jars with gas. 

Now, when water is opened out in this way by means of the 
battery, which adds nothing to it materially, which takes nothing 
from it materially (I mean no matter; I am not speaking of force), 
which adds no matter to the water, it is changed in this way — the gas 
which you saw burning a little while ago, called hydrogen, is evolved 
in large quantity, and the other gas, oxygen, is evolved in only half 
the quantity; so that these two areas represent water, and these are 
always the proportions between the two gases. 



Oxygen 88.9 

Hydrogen .... 11. i 

Water loo.o 



But oxygen is sixteen times the weight of the other — eight times 
as heavy as the particles of hydrogen in the water; and you therefore 
know that water is composed of nine parts by weight — one of hydro- 
gen and eight of oxygen; thus: 

Hydrogen 46.2 cubic inches =1 grain 

Oxygen 23.1 " " =8 grains 

Water {steam) 69.3 " " =9 grains 

Now Mr. Anderson has prepared some oxygen, and we will pro- 



I 
Hydrogen 


8 
Oxygen 


9 



CHEMICAL AFFINITY 49 

ceed to examine what is the character of this gas. First of all, you 
remember I told you that it does not burn, but that it affects the 
burning of other bodies. I will just set fire to the point of this little 
bit of wood, and then plunge it into the jar of oxygen, and you will 
see what this gas does in increasing the brilliancy of the combustion. 
It does not burn, it does not take fire as the hydrogen would; but 
how vividly the combustion of the match goes on! Again, if I were 
to take this wax taper and light it, and turn it upside down in the 
air, it would, in all probability, put itself out, owing to the wax run- 
ning down into the wick. [The lecturer here 
turned the lighted taper upside down, when in a 
few seconds it went out.] Now that will not 
happen in oxygen gas; you will see how differently 
it acts (Fig. 26). [The taper was again lighted, 
turned upside down, and then introduced into a 
jar of oxygen.] Look at that! See how the very 
wax itself burns, and falls down in a dazzling' 
stream of fire, so powerfully does the oxygen sup- ^ig. 26 

port combustion. Again, here is another experiment which will serve 
to illustrate the force, if I may so call it, of oxygen. I have here a 
circular flame of spirit of wine, and with it I am about to show you 
the way in which iron burns, because it will serve very well as a 
comparison between the effect produced by air and oxygen. If I take 
this ring flame, I can shake, by means of a sieve, the fine particles of 
iron filings through it, and you will see the way in which they burn. 
[The lecturer here shook through the flame some iron filings, which 
took fire and fell through with beautiful scintillations.] But if I now 
hold the flame over a jar of oxygen [the experiment was repeated 
over a jar of oxygen, when the combustion of the filings as they fell 
into the oxygen became almost insupportably brilliant], you see how 
wonderfully different the effect is in the jar, because there we have 
oxygen instead of common air. 




LECTURE IV 
CHEMICAL AFFINITY-HEAT 

WE shall have to pay a little more attention to the forces 
existing in water before we can have a clear idea on the 
subject. Besides the attraction which there is between its 
particles to make it hold together as a liquid or a solid, there is also 
another force, different from the former — one which, yesterday, by 
means of the voltaic battery, we overcame, drawing from the water 
two different substances, which, when heated by means of the electric 
spark, attracted each other, and rushed into combination to repro- 
duce water. Now I propose to-day to continue this subject, and trace 
the various phenomena of chemical affinity; and for this purpose, as 
we yesterday considered the character of oxygen, of which I have 
here two jars (oxygen being those particles derived from the water 
which enable other bodies to burn), we will now consider the other 
constituent of water, and, without embarrassing you too much with 
the way in which these things are made, I will proceed now to show 
you our common way of making hydrogen. (I called it hydrogen 
yesterday: it is so called because it helps to generate water.)* I put 
into this retort some zinc, water, and oil of vitriol, and immediately 
an action takes place, which produces an abundant evolution of gas, 
now coming over into this jar, and bubbling up in appearance 
exactly like the oxygen we obtained yesterday (Fig. 27). 

The processes, you see, are very different, though the result is the 
same, in so far as it gives us certain gaseous particles. Here, then, is 
the hydrogen. I showed you yesterday certain qualities of this gas; 
now let me exhibit you some other properties. Unlike oxygen, which 
is a supporter of combustion and will not burn, hydrogen itself is 
combustible. There is a jar full of it; and if I carry it along in this 
manner and put a light to it, I think you will see it take fire (Fig. 27) 
— not with a bright light; you will, at all events, hear it if you do not 

*'TSa)p, "water," and ytwahi, "I generate." 
50 



CHEMICAL AFFINITY 5 1 

see it. Now that is a body entirely diflerent from oxygen; it is ex- 
tremely light; for, although yesterday you saw twice as much of this 
hydrogen produced on the one side as on the other by the voltaic 




Fig. 27 

battery, it was only one-eighth the weight of the oxygen. I carry this 
jar upside down. Why.? Because I know that it is a very light body, 
and that it will continue in this jar upside down quite as effectually 
as the water will in that jar which is not upside down; and just as 
I can pour water from one vessel into another in the right position to 
receive it, so can I pour this gas from one 
jar into another when they are upside 
down. See what I am about to do. There 
is no hydrogen in this jar at present, but I 
will gently turn this jar of hydrogen up 
under this other jar (Fig. 28), and then 
we will examine the two. We shall see, 
on applying a light, that the hydrogen 
has left the jar in which it was at first, 
and has poured upward into the other, and there we shall find it. 
You now understand that we can have particles of very different 
kinds, and that they can have different bulks and weights; and there 
are two or three very interesting experiments which serve to illus- 
trate this. For instance, if I blow soap bubbles with the breath from 
my mouth, you will see them fall, because I fill them with common 
air, and the water which forms the bubble carries it down. But now, 
if I inhale hydrogen gas into my lungs (it does no harm to the lungs, 
although it does no good to them), see what happens. [The lecturer 
inhaled some hydrogen, and, after one or two ineffectual attempts, 




Fig. 28 



52 FARADAY 

succeeded in blowing a splendid bubble, which rose majestically and 
slowly to the ceiling of the theatre, where it burst.] That shows you 
very well how light a substance this is; for, notwithstanding all the 
heavy bad air from my lungs, and the weight of the bubble, you saw 
how it was carried up. I want you now to consider this phenomenon 
of weight as indicating how exceedingly different particles are one 

Jljdr 

Platinum. 



Water: 

o 




from the other; and I will take as illustrations these very common 
things, air, water, the heaviest body, platinum, and this gas, and 
observe how they differ in this respect; for if I take a piece of plat- 
inum of that size (Fig. 29), it is equal to the weight of portions of 
water, air, and hydrogen of the bulks I have represented in these 
spheres; and this illustration gives you a very good idea of the extraor- 
dinary difference with regard to the gravity of the articles having 
this enormous difference in bulk. [The following tabular statement 
having reference to this illustration appeared on the diagram board.] 



Hydrogen 


I 






Air 




I 




^^ 


Water 


11943 


829 


I 


Platinum 


256774 


1 7831 


21-5 



Whenever oxygen and hydrogen unite together they produce 
water, and you have seen the extraordinary difference between the 




CHEMICAL AFFINITY 53 

bulk and appearance of the water so produced and the particles of 
which it consists chemically. Now we have never yet been able to 
reduce either oxygen or hydrogen to the liquid state; and yet their 
first impulse, when chemically combined, is to take up first this 
liquid condition and then the solid condition. We never combine 
these different particles together without producing water; and it is 
curious to think how often you must have 
made the experiment of combining oxygen 
and hydrogen to form water without know- 
ing it. Take a candle, for instance, and a 
clean silver spoon (or a piece of clean tin 
will do), and, if you hold it over the flame, 
you immediately cover it with dew — not a 
smoke — which presently evaporates. This, 
perhaps, will serve to show it better. Mr. 
Anderson will put a candle under that jar, ''°' ^° 

and you will see how soon the water is produced (Fig. 30) . Look at 
that dimness on the sides of the glass, which will soon produce drops, 
and trickle down into the plate. Well, that dimness and these drops 
are water, formed by the union of the oxygen of the air with the 
hydrogen existing in the wax of which that candle is formed. 

And now, having brought you, in the first place, to the consider- 
ation of chemical attraction, I must enlarge your ideas so as to 
include all substances which have this attraction for each other; for 
it changes the character of bodies, and alters them in this way and 
that way in the most extraordinary manner, and produces other 
phenomena wonderful to think about. Here is some chlorate of 
potash, and there some sulphuret of antimony ('*). We will mix 
these two different sets of particles together and I want to show you, 
in a general sort of way, some of the phenomena which take place 
when we make different particles act together. Now I can make 
these bodies act upon each other in several ways. In this case I am 
going to apply heat to the mixture; but if I were to give a blow 
with a hammer, the same result would follow. [A lighted match 

" chlorate of potash and sulphuret of antimony. Great care must be taken in 
mixing these substances, as the mixture is dangerously explosive. They must be 
powdered separately and mixed together with a feather on a sheet ot paper, or by 
passing them several times through a small sieve. 



54 FARADAY 

was brought to the mixture, which immediately exploded with a 
sudden flash, evolving a dense white smoke.] There you see the 
result of the action of chemical affinity overcoming the attraction of 
cohesion of the particles. Again, here is a little sugar ("), quite a 
different substance from the black sulphuret of antimony, and you 
shall see what takes place when we put the two together. [The 
mixture was touched with sulphuric acid, when it took fire, and 
burnt gradually and with a brighter flame than in the former 
instance.] Observe this chemical affinity traveling about the mass, 
and setting it on fire, and throwing it into such wonderful agitation! 

I must now come to a few circumstances which require careful 
consideration. We have already examined one of the effects of this 
chemical affinity, but, to make the matter more clear, we must 
point out some others. And here are two salts dissolved in water('*). 
They are both colorless solutions, and in these glasses you can not 
see any difference between them. But if I mix them, I shall have 
chemical attraction take place. I will pour the two together into 
this glass, and you will at once see, I have no doubt, a certain amount 
of change. Look, they are already becoming milky, but they are 
sluggish in their action — not quick as the others were — for we have 
endless varieties of rapidity in chemical action. Now, if I mix them 
together, and stir them so as to bring them properly together, you 
will soon see what a different result is produced. As I mix them they 
get thicker and thicker, and you see the liquid is hardening and 
stiffening, and before long I shall have it quite hard; and before 
the end of the lecture it will be a solid stone — a wet stone, no doubt, 
but more or less solid — in consequence of the chemical affinity. Is not 
this changing two liquids into a solid body a wonderful manifesta- 
tion of chemical affinity ? 

There is another remarkable circumstance in chemical affinity, 
which is, that it is capable of either waiting or acting at once. And 
this is very singular, because we know of nothing of the kind in 
the forces either of gravitation or cohesion. For instance: here are 

"The mixture of chlorate of potash and sugar does not require the same pre- 
cautions. They may be rubbed together in a pestle and mortar without fear. One 
part of chlorate of potash and three parts of sugar will answer. The mixture need 
only be touched with a glass rod dipped in oil of vitriol. 

'* Two salts dissolved in water. Sulphate of soda and chloride of calcium. The 
solutions must be saturated for the experiment to succeed well. 



CHEMICAL AFFINITY 55 

some oxygen particles, and here is a lump of carbon particles. I am 
going to put the carbon particles into the oxygen; they can act, but 
they do not — they are just Uke this unhghted candle. It stands here 
quietly on the table, waiting until we want to light it. But it is not 
so in this other case: here is a substance, gaseous like the oxygen, 
and if I put these particles of metal into it the two combine at once. 
The copper and the chlorine unite by their power of chemical affinity, 
and produce a body entirely unlike either of the substances used. 
And in this other case, it is not that there is any deficiency of affinity 
between the carbon and oxygen, for the moment I choose to put 
them in a condition to exert their affinity, you will see the difference. 
[The piece of charcoal was ignited, and introduced into the jar of 
oxygen, when the combustion proceeded with vivid scintillations.] 
Now this chemical action is set going exactly as it would be if I 
had lighted the candle, or as it is when the servant puts coals on and 
lights the fire: the substances wait until we do something which is 
able to start the action. Can any thing be more beautiful than this 
combustion of charcoal in oxygen? You must understand that each 
of these little sparks is a portion of the charcoal, or the bark of the 
charcoal thrown off white hot into the oxygen, and burning in it 
most brilliantly, as you see. And now let me tell you another thing, 
or you will go away with a very imperfect notion of the powers 
and effects of this affinity. There you see some charcoal burn- 
ing in oxygen. Well, a piece of lead will burn in oxygen just 
as well as the charcoal does, or indeed better, for absolutely that 
piece of lead will act at once upon the oxygen as the copper did in 
the other vessel with regard to the chlorine. And here, also, a piece 
of iron — if I light it and put it into the oxygen, it will burn away 
just as the carbon did. And I will take some lead, and show you that 
it will burn in the common atmospheric oxygen at the ordinary 
temperature. These are the lumps of lead which you remember we 
had the other day — the two pieces which clung together. Now 
these pieces, if I take them to-day and press them together, will not 
stick, and the reason is that they have attracted from the atmosphere 
a part of the oxygen there present, and have become coated as with 
a varnish by the oxide of lead, which is formed on the surface by a 
real process of combustion or combination. There you see the iron 



56 FARADAY 

burning very well in oxygen, and I will tell you the reason why those 
scissors and that lead do not take fire while they are lying on the 
table. Here the lead is in a lump, and the coating of oxide remains 
on its surface, while there you see the melted oxide is clearing itself 
off from the iron, and allowing more and more to go on burning. 
In this case, however, [holding up a small glass tube containing 
lead pyrophorus(")], the lead has been very carefully produced in 
fine powder, and put into a glass tube, and hermetically sealed so as 
to preserve it, and I expect you will see it take fire at once. This has 
been made about a month ago, and has thus had time to sink down 
to its normal temperature; what you see, therefore, is the result 
of chemical affinity alone. [The tube was broken at the end, and 
the lead poured out on to a piece of paper, whereupon it immediately 
took fire.] Look! look at the lead burning! Why, it has set fire to 
the paper! Now that is nothing more than the common affinity 
always existing between very clean lead and the atmospheric oxygen; 
and the reason why this iron does not burn until it is made red hot 
is because it has got a coating of oxide about it, which stops the action 
of the oxygen — putting a varnish, as it were, upon its surface, as we 
varnish a picture — absolutely forming a substance which prevents 
the natural chemical affinity between the bodies from acting. 

I must now take you a little farther in this kind of illustration, or 
consideration I would rather call it, of chemical affinity. This attrac- 
tion between different particles exists also most curiously in cases 
where they are previously combined with other substances. Here 
is a little chlorate of potash containing the oxygen which we found 
yesterday could be procured from it; it contains the oxygen there 
combined and held down by its chemical affinity with other things, 
but still it can combine with sugar, as you saw. This affinity can thus 
act across substances, and I want you to see how curiously what we 
call combustion acts with respect to this force of chemical affinity. 
If I take a piece of phosphorus and set fire to it, and then place a 
jar of air over the phosphorus, you see the combustion which we 
are having there on account of chemical affinity (combustion being 
in all cases the result of chemical affinity). The phosphorus is 

'* Lead pyrophorus. This is tartrate of lead which has been heated in a glass tube 
to dull redness as long as vapors are emitted. As soon as they cease to be evolved 
the end of the tube is sealed, and it is allowed to cool. 



CHEMICAL AFFINITY 57 

escaping in that vapor, whicii will condense into a snowlike mass at 
the close of the lecture. But suppose I limit the atmosphere, what 
then? why, even the phosphorus will go out. Here is a piece o£ 
camphor, which will burn very well in the atmosphere, and even on 
water it will float about and burn, by reason of some of its particles 
gaining access to the air. But if I limit the quantity of air by placing 
a jar over it, as I am now doing, you will soon find the camphor 
will go out. Well, why does it go out ? not for want of air, for there 
is plenty of air remaining in the jar. Perhaps you will be shrewd 
enough to say for want of oxygen. 

This, therefore, leads us to the inquiry as to whether oxygen can 
do more than a certain amount of work. The oxygen there (Fig. 30) 
can not go on burning an unlimited quantity of candle, for that has 
gone out, as you see; and its amount of chemical attraction or 
affinity is just as strikingly limited: it can no more be fallen short 
of or exceeded than can the attraction of gravitation. You might 
as soon attempt to destroy gravitation, or weight, or all things that 
exist, as to destroy the exact amount of force exerted by this oxygen. 
And when I pointed out to you that eight by weight of oxygen to 
one by weight of hydrogen went to form water, I meant this, that 
neither of them would combine in different proportions with the 
other, for you can not get ten of hydrogen to combine with six of 
oxygen, or ten of oxygen to combine with six of hydrogen; it must 
be eight of oxygen and one of hydrogen. Now suppose I limit the 
action in this way : this piece of cotton wool burns, as you see, very 
well in the atmosphere; and I have known of cases of cotton-mills 
being fired as if with gunpowder through the very finely divided 
particles of cotton being diffused through the atmosphere in the 
mill, when it has sometimes happened that a flame has caught these 
raised particles, and it has run from one end of the mill to the 
other and blown it up. That, then, is on account of the affinity 
which the cotton has for the oxygen; but suppose I set fire to this 
piece of cotton v/hich is rolled up tightly; it does not go on burning, 
because I have limited the supply of oxygen, and the inside is pre- 
vented from having access to the oxygen, just as it was in the case 
of the lead by the oxide. But here is some cotton which has been 
imbued with oxygen in a certain manner. I need not trouble you 



58 FARADAY 

now with the way it is prepared; it is called guncottonC""). See how 
that burns [setting fire to a piece]; it is very different from the 
other, because the oxygen which must be present in its proper 
amount is put there beforehand. And I have here some pieces of 
paper which are prepared like the guncotton(^'), and imbued with 
bodies containing oxygen. Here is some which has been soaked in 
nitrate of strontia: you will see the beautiful red color of its flame; 
and here is another which I think contains baryta, which gives that 
fine green light; and I have here some more which has been soaked 
in nitrate of copper : it does not burn quite so brightly, but still very 
beautifully. In all these cases the combustion goes on independent 
of the oxygen of the atmosphere. And here we have some gun- 
powder put into a case, in order to show that it is capable of burning 
under water. You know that we put it into a gun, shutting off the 
atmosphere with shot, and yet the oxygen which it contains supplies 
the particles with that without which chemical action could not 
proceed. Now I have a vessel of water here, and am going to make 
the experiment of putting this fuse under the water, and you will 
see whether that water can extinguish it; here it is burning out of 
the water, and there it is burning under the water; and so it will 
continue until exhausted, and all by reason of the requisite amount 
of oxygen being contained within the substance. It is by this kind 
of attraction of the different particles one to the other that we are 
enabled to trace the laws of chemical affinity, and the wonderful 
variety of the exertions of these laws. 

Now I want you to observe that one great exertion of this power 
which is known as chemical affinity is to produce Heat and light; 
you know, as a matter of fact, no doubt, that when bodies burn they 
give out heat, but it is a curious thing that this heat does not con- 
tinue; the heat goes away as soon as the action stops, and you see, 
thereby, that it depends upon the action during the time it is going 

^^ Guncotton is made by immersing cotton wool in a mixture of sulphuric acid and 
the strongest nitric acid or of sulphuric acid and nitrate of potash. 

2' Paper prepared lik.e guncotton. It should be bibulous paper, and must be soaked 
for ten minutes in a mixture of ten parts, by measure, of oil of vitriol with five parts 
of strong fuming nitric acid. The paper must afterward be thoroughly washed with 
warm distilled water, and then carefully dried at a gentle heat. The paper is then 
saturated with chlorate of strontia, or chlorate of baryta, or nitrate of copper, by im- 
mersion in a warm solution of these salts. (See Chemical News, vol. i., p. 36.) 



CHEMICAL AFFINITY 59 

on. It is not so with gravitation; this force is continuous, and is just 
as effective in making that lead press on the table as it was when it 
first fell there. Nothing occurs there which disappears when the 
action of falling is over; the pressure is upon the table, and will 
remain there until the lead is removed; whereas, in the action of 
chemical affinity to give light and heat, they go away immediately 
the action is over. This lamp seems to evolve heat and light con- 
tinuously, but it is owing to a constant stream of air coming into it 
on all sides, and this work of producing light and heat by chemical 
affinity will subside as soon as the stream of air is interrupted. What, 
then, is this curious condition of heat? Why, it is the evolution of 
another power of matter — of a power new to us, and which we must 
consider as if it were now for the very first time brought under our 
notice. What is heat ? We recognize heat by its power of liquefying 
solid bodies and vaporizing liquid bodies; by its power of setting 
in action, and very often overcoming, chemical affinity. Then how 
do we obtain heat? We obtain it in various ways; most abundantly 
by means of the chemical affinity we have just before been speaking 
about, but we can also obtain it in many other ways. Friction will 
produce heat. The Indians rub pieces of wood together until they 
make them hot enough to take fire; and such things have been 
known as two branches of a tree rubbing together so hard as to set 
the tree on fire. I do not suppose I shall set these two pieces of wood 
on fire by friction, but I can readily produce heat enough to ignite 
some phosphorus. [The lecturer here rubbed two pieces of cedar 
wood strongly against each other for a minute, and then placed on 
them a piece of phosphorus, which immediately took fire.] And if 
you take a smooth metal button stuck on a cork, and rub it on a 
piece of soft deal wood, you will make it so hot as to scorch wood 
and paper, and burn a match. 

I am now going to show you that we can obtain heat, not by 
chemical affinity alone, but by the pressure of air. Suppose I take a 
pellet of cotton and moisten it with a little ether, and put it into a 
glass tube (Fig. 31), and then take a piston and press it down sud- 
denly, I expect I shall be able to burn a little of that ether in the 
vessel. It wants a suddenness of pressure, or we shall not do what 
we require. [The piston was forcibly pressed down, when a flame, 



6o FARADAY 

due to the combustion of the ether, was visible in the lower part of 
the syringe.] All we want is to get a little ether in vapor, and give 
fresh air each time, and so we may go on again and again, getting 
heat enough by the compression of air to fire the ether-vapor. 

This, then, I think, will be sufficient, accompanied with all you 
have previously seen, to show you how we procure heat. And now 
for the effects of this power. We need not consider many of 
them on the present occasion, because, when you have seen its 
power of changing ice into water and water into steam, you 
have seen the two principal results of the application of heat. 
I want you now to see how it expands all bodies — all bodies 
but one, and that under limited circumstances. Mr. Anderson 
will hold a lamp under that retort, and you will see, the 
moment he does so, that the air will issue abundantly from 
the neck which is under water, because the heat which he 
^' applies to the air causes it to expand. And here is a brass rod 
(Fig. 32) which goes through that hole, and fits also accurately into 
this gauge; but if I make it warm with this spirit lamp, it will only 
go in the gauge or through the hole with difficulty; and if I were to 
put it into boiling water it would not go through at all. Again, as 
soon as the heat escapes from bodies, they collapse: see how the air 



3) 



( ? -Q 



Fig. 32 

is contracting in the vessel now that Mr. Anderson has taken away 
his lamp; the stem of it is filling with water. Notice too, now, that 
although I cannot get the tube through this hole or into the gauge, 
the moment I cool it, by dipping it into water, it goes through with 
perfect facility, so that we have a perfect proof of this power of heat 
to contract and expand bodies. 



LECTURE V 
MAGNETISM— ELECTRICITY 

I WONDER whether we shall be too deep to-day or not. Remem- 
ber that we spoke of the attraction by gravitation of all bodies 
to all bodies by their simple approach. Remember that we 
spoke of the attraction of particles of the same kind to each other — 
that power which keeps them together in masses — iron attracted to 
iron, brass to brass, or water to water. Remember that we found, on 
looking into water, that there were particles of two different kinds 
attracted to each other; and this was a great step beyond the first 
simple attraction of gravitation, because here we deal with attraction 
between different kinds of matter. The hydrogen could attract the 
oxygen and reduce it to water, but it could not attract any of its 
own particles, so that there we obtained a first indication of the 
existence of two attractions. 

To-day we come to a kind of attraction even more curious than 
the last, namely, the attraction which we find to be of a double 
nature — of a curious and dual nature. And I want, first of all, to 
make the nature of this doubleness clear to you. Bodies are some- 
times endowed with a wonderful attraction, which is not found in 
them in their ordinary state. For instance, here is a piece of shellac, 
having the attraction of gravitation, having the attraction of cohesion, 
and if I set fire to it, it would have the attraction of chemical affinity 
tb the oxygen in the atmosphere. Now all these powers we find in 
it as if they were parts of its substance; but there is another property 
which I will try and make evident by means of this ball, this bubble 
of air [a light India-rubber ball, inflated and suspended by a thread]. 
There is no attraction between this ball and this shellac at present; 
there may be a little wind in the rooms slightly moving the ball 
about, but there is no attraction. But if I rub the shellac with a 
piece of flannel [rubbing the shellac, and then holding it near the 
ball], look at the attraction which has arisen out of the shellac simply 

6i 



62 FARADAY 

by this friction, and which I may take away as easily by drawing 
it gently through my hand. [The lecturer repeated the experiment 
of exciting the shellac, and then removing the attractive power by 
drawing it through his hand.] Again, you will see I can repeat this 
experiment with another substance; for if I take a glass rod, and 
rub it with a piece of silk covered with what we call amalgam, look 
at the attraction which it has; how it draws the ball toward it; and 
then, as before, by quietly rubbing it through the hand, the attraction 
will be all removed again, to come back by friction with this silk. 

But now we come to another fact. I will take this piece of shellac, 
and make it attractive by friction; and remember that, whenever 
we get an attraction of gravity, chemical affinity, adhesion, or elec- 
tricity (as in this case), the body which attracts is attracted also, and 
just as much as that ball was attracted by the shellac, the shellac was 
attracted by the ball. Now I will suspend this piece of excited shellac 
in a little paper stirrup, in this way (Fig. 33), in order to make it 
move easily, and I will take another piece of shellac, and, after 




rubbing it with flannel, will bring them near together: you will 
think that they ought to attract each other; but now what happens.'' 
It does not attract; on the contrary, it very strongly repels, and I 
can thus drive it round to any extent. These, therefore, repel each 
other, although they are so strongly attractive — repel each other to 
the extent of driving this heavy piece of shellac round and round 
in this way. But if I excite this piece of shellac as before, and take 
this piece of glass and rub it with silk, and then bring them near, 



ELECTRICITY 63 

what think you will happen? [The lecturer held the excited glass 
near the excited shellac, when they attracted each other strongly.] 
You see, therefore, what a difference there is between these two 
attractions; they are actually two \inds of attraction concerned in 
this case, quite different to any thing we have met with before, but 
the force is the same. We have here, then, a double attraction — a 
dual attraction or force — one attracting and the other repelling. 

Again, to show you another experiment which will help to make 
this clear to you: Suppose I set up this rough indicator again [the 




Fig. 34 

excited shellac suspended in the stirrup]: it is rough, but delicate 
enough for my purpose; and suppose I take this other piece of 
shellac, and take away the power, which I can do by drawing it 
gently through the hand; and suppose I take a piece of flannel 
(Fig. 34), which I have shaped into a cap for it and made dry. I 
will put this shellac into the flannel, and here comes out a very 
beautiful result. I will rub this shellac and the flannel together 
(which I can do by twisting the shellac round), and leave them in 
contact; and then if I ask, by bringing them near our indicator, 
what is the attractive force.? it is nothing; but if I take them apart, 
and then ask what will they do when they are separated? why, the 
shellac is strongly repelled, as it was before, but the cap is strongly 
attractive; and yet, if I bring them both together again, there is no 
attraction; it has all disappeared [the experiment was repeated]. 



64 FARADAY 

Those two bodies, therefore, still contain this attractive power; when 
they were parted, it was evident to your senses that they had it, 
though they do not attract when they are together. 

This, then, is sufficient, in the outset, to give you an idea o£ the 
nature of the force which we call Electricity. There is no 




Fig. 35 

end to the things from which you can evolve this power. When you 
go home, take a stick of sealing-wax — I have rather a large stick, but 
a smaller one will do — and make an indicator of this sort (Fig. 35). 
Take a watch-glass (or your watch itself will do; you only want 
something which shall have a round face) ; and now, if you place 
a piece of flat glass upon that, you have a very easily moved centre; 
and if I take this lath and put it on the flat glass (you see I am 
searching for the centre of gravity of this lath; I want to balance it 
upon the watch-glass), it is very easily moved round; and if I take 
this piece of sealing-wax and rub it against my coat, and then try 
whether it is attractive [holding it near the lath], you see how strong 
the attraction is; I can even draw it about. Here, then, you have a 
very beautiful indicator, for I have, with a small piece of sealing-wax 

and my coat, pulled round a plank of 
that kind, so you need be in no want 
of indicators to discover the presence 
of this attraction. There is scarcely a 
substance which we may not use. 
Here are some indicators (Fig. 36). I 
^°' ^ bend round a strip of paper into a 

hoop, and we have as good an indicator as can be required. See how 
it rolls along, traveling after the sealing-wax! If I make them smaller, 
of course we have them running faster, and sometimes they are actu- 
ally attracted up into the air. Here, also, is a Httle collodion balloon. 
It is so electrical that it will scarcely leave my hand unless to go to the 




ELECTRICITY 65 

Other. See how curiously electrical it is; it is hardly possible for me 
to touch it without making it electrical; and here is a piece which 
clings to any thing it is brought near, and which it is not easy to lay 
down. And here is another substance, gutta-percha, in thin strips: 
it is astonishing how, by rubbing this in your hands, you make it 
electrical; but our time forbids us to go farther into this subject at 
present; you see clearly there are two kinds of electricities which 
may be obtained by rubbing shellac with flannel or glass with silk. 
Now there are some curious bodies in nature (of which I have 
two specimens on the table) which are called magnets or loadstones; 
ores of iron, of which there is a great deal sent from Sweden. They 
have the attraction of gravitation, and attraction of cohesion, and 
certain chemical attraction; but they also have a great attractive 
power, for this little key is held up by this stone. Now that is not 
chemical attraction; it is not the attraction of chemical affinity, or 
of aggregation of particles, or of cohesion, or of electricity (for it will 
not attract this ball if I bring it near it), but it is a separate and 
dual attraction, and, what is more, one which is not readily removed 
from the substance, for it has existed in it for ages and ages in the 
bowels of the earth. Now we can make artificial magnets (you 
will see me to-morrow make artificial magnets of extraordinary 
power) . And let us take one of these artificial magnets and examine 
it, and see where the power is in the mass, and whether it is a dual 
power. You see it attracts these keys, two or three in succession, 
and it will attract a very large piece of iron. That, then, is a very 
different thing indeed to what you saw in the case of the shellac, for 
that only attracted a light ball, but here I have several ounces of iron 
held up. And if we come to examine this attraction a little more 
closely, we shall find it presents some other remarkable differences; 
first of all, one end of this bar (Fig. 37) attracts this key, but the 
middle does not attract. It is not, then, the whole of the substance 
which attracts. If I place this little key in the middle it does not 
adhere; but if I place it there, a little nearer the end, it does, though 
feebly. Is it not, then, very curious to find that there is an attractive 
power at the extremities which is not in the middle — to have thus 
in one bar two places in which this force of attraction resides ? If I 
take this bar and balance it carefully on a point, so that it will be 



66 FARADAY 

free to move round, I can try what action this piece of iron has on it. 
Well, it attracts one end, and it also attracts the other end, just as 
you saw the shellac and the glass did, with the exception of its not 
attracting in the middle. But if now, instead of a piece of iron, I 
take a magnet, and examine it in a similar way, you see that one of 
its ends repels the suspended magnet; the force, then, is no longer 
attraction, but repulsion; but, if I take the other end of the magnet 
and bring it near, it shows attraction again. 

You will see this better, perhaps, by another kind of experiment. 
Here (Fig. 38) is a little magnet, and I have colored the ends difler- 





FiG. 37 Fig. 38 

ently, so that you may distinguish one from the other. Now this 
end (S) of the magnet (Fig. 37) attracts the uncolored end of the 
little magnet. You see it pulls toward it with great power; and, as I 
carry it round, the uncolored end still follows. But now, if I gradu- 
ally bring the middle of the bar magnet opposite the uncolored end 
of the needle, it has no effect upon it, either of attraction or 
repulsion, until, as I come to the opposite extremity (N), you 
see that it is the colored end of the needle which is pulled toward it. 
We are now, therefore, dealing with two kinds of power, attracting 
different ends of the magnet — a double power, already existing in 
these bodies, which takes up the form of attraction and repulsion. 
And now, when I put up this label with the word Magnetism, you 
will understand that it is to express this double power. 

Now with this loadstone you may make magnets artificially. 
Here is an artificial magnet (Fig. 39) in which both ends have been 
brought together in order to increase the attraction. This mass will 
lift that lump of iron, and, what is more, by placing this \eeper, 
as it is called, on the top of the magnet, and taking hold of the 




Fig. 39 



ELECTRICITY 67 

handle, it will adhere sufficiently strongly to allow itself to be lifted 
up, so wonderful is its power of attraction. If you take a needle, 
and just draw one of its ends along one extremity of the magnet, 
and then draw the other end along the other extremity, and then 
gently place it on the surface of some water (the needle 
will generally float on the surface, owing to the slight 
greasiness communicated to it by the fingers), you will 
be able to get all the phenomena of attraction and repul- 
sion by bringing another magnetized needle near to it. 

I want you now to observe that, although I have shown 
you in these magnets that this double power becomes evi- 
dent principally at the extremities, yet the whole of the 
magnet is concerned in giving the power. That will at 
first seem rather strange; and I must therefore show you 
an experiment to prove that this is not an accidental matter, but that 
the whole of the mass is really concerned in this force, just as in 
falling the whole of the mass is acted upon by the force of gravita- 
tion. I have here (Fig. 40) a steel bar, and I am going to make it a 
magnet by rubbing it on the large magnet- (Fig. 39). I have now 
made the two ends magnetic in opposite ways. I do not at present 
know one from the other, but we can soon find out. You see, when 
I bring it near our magnetic needle (Fig. 38), one end repels and 
the other attracts; and the middle will neither attract nor repel — ^it 
can not, because it is halj way between the two ends. But now, if I 
break out that piece («, s), and then examine it, see how strongly 
one end («) pulls at this end (S, Fig. 38), and how it repels the 
other end (N). And so it can be shown that every part of the magnet 
contains this power of attraction and repul- 
sion, but that the power is only rendered evi- 
dent at the end of the mass. You will 



N 



Fig. 40 



understand all this in a little while; but what 
you have now to consider is that every part of this steel is in itself a 
magnet. Here is a little fragment which I have broken out of the 
very centre of the bar, and you will still see that one end is attrac- 
tive and the other is repulsive. Now is not this power a most won- 
derful thing? And very strange, the means of taking it from one 
substance and bringing it to other matters. I can not make a piece 



68 FARADAY 

of iron or any thing else heavier or lighter than it is; its cohesive 
power it must and does have; but, as you have seen by these experi- 
ments, we can add or subtract this power of magnetism, and almost 
do as we like with it. 

And now we will return for a short time to the subject treated of 
at the commencement of this lecture. You see here (Fig. 41) a large 
machine arranged for the purpose of rubbing glass with silk, and 




Fig. 41 

for obtaining the power called electricity; and the moment the 
handle of the machine is turned a certain amount of electricity is 
evolved, as you will see by the rise of the little straw indicator (at A) . 
Now I know, from the appearance of repulsion of the pith ball at 
the end of the straw, that electricity is present in those brass con- 
ductors (BB), and I want you to see the manner in which that 
electricity can pass away [touching the conductor (B) with his 
finger, the lecturer drew a spark from it, and the straw electrometer 
immediately fell]. There, it has all gone; and that I have really 
taken it away you shall see by an experiment of this sort. If I hold 
this cylinder of brass by the glass handle, and touch the conductor 
with it, I take away a little of the electricity. You see the spark in 
which it passes, and observe that the pith-ball indicator has fallen 
a little, which seems to imply that so much electricity is lost; but it 
is not lost; it is here in this brass, and I can take it away and carry 
it about, not because it has any substance of its own, but by some 



ELECTRICITY 69 

Strange property which we have not before met with as belonging 
to any other force. Let us see whether we have it here or not. [The 
lecturer brought the charged cylinder to a jet from which gas was 
issuing; the spark was seen to pass from the cylinder to the jet, but 
the gas did not light.] Ah! the gas did not light, but you saw the 
spark; there is, perhaps, some draught in the room which blew the 
gas on one side, or else it would light; we will try this experiment 
afterward. You see from the spark that I can transfer the power 
from the machine to this cylinder, and then carry it away and give 
it to some other body. 

You know very well, as a matter of experiment, that we can trans- 
fer the power of heat from one thing to another; for if I put my 
hand near the fire it becomes hot. I can show you this by placing 
before us this ball, which has just been brought red-hot from the 
fixe. If I press this wire to it some of the heat will be transferred 
from the ball, and I have only now to touch this piece of gun-cotton 
with the hot wire, and you see how I can transfer the heat from the 
ball to the wire, and from the wire to the cotton. So you see that some 
powers are transferable, and others are not. Observe how long the 
heat stops in this ball. I might touch it with the wire or wdth my 
finger, and if I did so quickly I should merely burn the surface of 
the skin; whereas, if I touch that cylinder, however rapidly, with my 
finger, the electricity is gone at once — dispersed on the instant, in a 
manner wonderful to think of. 

I must now take up a little of your time in showing you the 
manner in which these powers are transferred from one thing to 
another; for the manner in which force may be conducted or trans- 
mitted is extraordinary, and most essential for us to understand. 
Let us see in what manner these powers travel from place to place. 
Both heat and electricity can be conducted; and here is an arrange- 
ment I have made to show how the former can travel. It consists of 
a bar of copper (Fig. 42) ; and if I take a spirit lamp (this is one way 
of obtaining the power of heat) and place it under that little chim- 
ney, the flame will strike against the bar of copper and keep it hot. 
Now you are aware that power is being transferred from the flame 
of that lamp to the copper, and you will see by-and-by that it is being 
conducted along the copper from particle to particle; for, inasmuch 



70 FARADAY 

as I have fastened these wooden balls by a little wax at particular 
distances from the point where the copper is first heated, first one 
ball will fall and then the more distant ones, as the heat travels 
along, and thus you will learn that the heat travels gradually through 
the copper. You will see that this is a very slow conduction of power 
as compared with electricity. If I take cylinders of wood and metal, 
joined together at the ends, and wrap a piece of paper round, and 
then apply the heat of this lamp to the place where the metal and 
wood join, you will see how the heat will accumulate where the 




Fig. 42 

wood is, and burn the paper with which I have covered it; but where 
the metal is beneath, the heat is conducted away too fast for the 
paper to be burned. And so, if I take a piece of wood and a piece 
of metal joined together, and put it so that the flame shall play 
equally both upon one and the other, we shall soon find that the 
metal will become hot before the wood; for if I put a piece of phos- 
phorus on the wood and another piece on the copper, you will find 
that the phosphorus on the copper will take fire before that on the 
wood is melted; and this shows you how badly the wood conducts 
heat. But with regard to the traveUng of electricity from place to 
place, its rapidity is astonishing. I will, first of all, take these pieces 
of glass and metal, and you will soon understand how it is that the 
glass does not lose the power which it acquired when it is rubbed 
by the silk; by one or two experiments I will show you. If I take 
this piece of brass and bring it near the machine, you see how the 
electricity leaves the latter and passes to the brass cyUnder. And 
again: if I take a rod of metal and touch the machine with it, I 
lower the indicator; but when I touch it with a rod of glass, no 




ELECTRICITY 7I 

power is drawn away, showing you that the electricity is conducted 
by the glass and the metal in a manner entirely different; and, to 
make you see that more clearly, we will take one of our Leyden 
jars. Now I must not embarrass your minds with this subject too 
much, but if I take a piece of metal and bring it against the knob 
at the top and the metallic coating at the bottom, you will see the 
electricity passing through the air as a brilliant spark. It takes no 
sensible time to pass through this; and if I were to take a long 
metallic wire, no matter what the length, at least as far as we are 
concerned, and if I make one end of it touch the outside, and the 
other touch the knob at the top, see how 
the electricity passes! It has flashed in- 
stantaneously through the whole length 
of this wire. Is not this different from 
the transmission of heat through this 
copper bar (Fig. 42) which has taken a 
quarter of an hour or more to reach the 
first ball ? 

Here is another experiment for the 
purpose of showing the conductibility of 
this power through some bodies and not 
through others. Why do I have this 
arrangement made of brass? [pointing 
to the brass work of the electrical ma- 
chine, Fig. 41]. Because it conducts elec- 
tricity. And why do I have these col- 
umns made of glass? Because they 
obstruct the passage of electricity. And 
why do I put that paper tassel (Fig. 43) 
at the top of the pole, upon a glass rod, 
and connect it with this machine by 
means of a wire ? You see at once that 
as soon as the handle of the machine is 
turned, the electricity which is evolved travels along this wire and up 
the wooden rod, and goes to the tassel at the top, and you see the 
power of repulsion with which it has endowed these strips of paper, 
each spreading outward to the ceiling and sides of the room. The out- 




FiG. 43 



72 FARADAY 

side of that wire is covered with gutta-percha; it would not serve 
to keep the force from you when touching it with your hands, 
because it would burst through; but it answers our purpose for the 
present. And so you perceive how easily I can manage to send this 
power of electricity from place to place by choosing the materials 
which can conduct the power. Suppose I want to fire a portion of 
gunpowder, I can readily do it by this transferable power of elec- 
tricity. I will take a Leyden jar, or any other arrangement which 
gives us this power, and arrange wires so that they may carry the 
power to the place I wish; and then placing a little gunpowder on 
the extremities of the wires, the moment I make the connection by 
this discharging rod I shall fire the gunpowder [the connection was 
made and the gunpowder ignited]. And if I were to show you a 
stool like this, and were to explain to you its construction, you could 
easily understand that we use glass legs because these are capable 
of preventing the electricity from going away to the earth. If, there- 
fore, I were to stand on this stool, and receive the electricity through 
this conductor, I could give it to anything that I touched. [The 
lecturer stood upon the insulating stool, and placed himself in con- 
nection with the conductor of the machine.] Now I am electrified; 
I can feel my hair rising up, as the paper tassel did just now. Let us 
see whether I can succeed in lighting gas by touching the jet with 
my finger. [The lecturer brought his finger near a jet from which 
gas was issuing, when, after one or two attempts, the spark which 
came from his iinger to the jet set fire to the gas.] You now see how 
it is that this power of electricity can be transferred from the matter 
in which it is generated, and conducted along wires and other bodies, 
and thus be made to serve new purposes, utterly unattainable by the 
powers we have spoken of on previous days; and you will not now 
be at a loss to bring this power of electricity into comparison with 
those which we have previously examined, and to-morrow we shall 
be able to go farther into the consideration of these transferable 
powers. 



LECTURE VI 
THE CORRELATION OF THE PHYSICAL FORCES 

WE have frequently seen, during the course of these lectures, 
that one of those powers or forces of matter, of which I 
have written the names on that board, has produced results 
which are due to the action of some other force. Thus you have 
seen the force of electricity acting in other ways than in attracting; 
you have also seen it combine matters together or disunite them by 
means of its action on the chemical force; and in this case, therefore, 
you have an instance in which these two powers are related. But 
we have other and deeper relations than these; we have not merely 
to see how it is that one power affects another — how the force of 
heat affects chemical affinity, and so forth, but we must try and 
comprehend what relation they bear to each other, and how these 
powers may be changed one into the other; and it will to-day require 
all my care, and your care too, to make this clear to your minds. 
I shall be obliged to confine myself to one or two instances, because 
to take in the whole extent of this mutual relation and conversion 
of forces would surpass the human intellect. 

In the first place, then, here is a piece of fine zinc foil, and if I cut 
it into narrow strips and apply to it the power of heat, admitting the 
contact of air at the same time, you will find that it burns; and then, 
seeing that it burns, you will be prepared to say that there is chemi- 
cal action taking place. You see all I have to do is to hold the piece 
of zinc at the side of the flame, so as to let it get heated, and yet to 
allow the air which is flowing into the flame from all sides to have 
access to it; there is the piece of zinc burning just like a piece of 
wood, only brighter. A part of the zinc is going up into the air 
in the form of that white smoke, and part is falling down on to the 
table. This, then, is the action of chemical affinity exerted between 
the zinc and the oxygen of the air. I will show you what a curious 
kind of affinity this is by an experiment which is rather striking 

73 



74 FARADAY 

when seen for the first time. I have here some iron fiUngs and gun- 
powder, and will mix them carefully together, with as little rough 
handling as possible; now we will compare the combustibility, so to 
speak, of the two. I will pour some spirit of wine into a basin and 
set it on fire; and, having our flame, I will drop this mixture of 
iron filings and gunpowder through it, so that both sets of particles 
will have an equal chance of burning. And now tell me which of 
them it is that burns? You see a plentiful combustion of the iron 
filings; but I want you to observe that, though they have equal 
chances of burning, we shall find that by far the greater part of the 
gunpowder remains untouched; I have only to drain off this spirit 
of wine, and let the powder which has gone through the flame dry, 
which it will do in a few minutes, and I will then test it with a 
lighted match. So ready is the iron to burn, that it takes, under 
certain circumstances, even less time to catch fire than gunpowder. 
[As soon as the gunpowder was dry, Mr. Anderson handed it to the 
lecturer, who apphed a lighted match to it, when a sudden flash 
showed how large a proportion of gunpowder had escaped com- 
bustion when falling through the flame of alcohol.] 

These are all cases of chemical affinity, and I show them to make 
you understand that we are about to enter upon the consideration of 
a strange kind of chemical affinity, and then to see how far we are 
enabled to convert this force of affinity into electricity or magnetism, 
or any other of the forces which we have discussed. Here is some 
zinc (I keep to the metal zinc, as it is very useful for our purpose), 
and I can produce hydrogen gas by putting the zinc and sulphuric 
acid together, as they are in that retort; there you see the mixture 
which gives us hydrogen — the zinc is pulling the water to pieces and 
setting free hydrogen gas. Now we have learned by experience that 
if a little mercury is spread over that zinc, it does not tal{e away its 
power of decomposing the water, but modifies it most curiously. See 
how that mixture is now boiling; but when I add a little mercury 
to it the gas ceases to come off. We have now scarcely a bubble of 
hydrogen set free, so that the action is suspended for the time. We 
have not destroyed the power of chemical affinity, but modified it 
in a wonderful and beautiful manner. Here are some pieces of zinc 
covered with mercury exactly in the same way as the zinc in that 



THE CORRELATION OF THE PHYSICAL FORCES 75 

retort is covered; and if I put this plate into sulphuric acid I get no 
gas, but this most extraordinary thing occurs, that i£ I introduce 
along with the zinc another metal which is not so combustible, then 
I reproduce all the action. I am now going to put to the amalga- 
mated zinc in this retort some portions of copper wire (copper not 
being so combustible a metal as the zinc), and observe how I get 
hydrogen again, as in the first instance; there, the bubbles are com- 
ing over through the pneumatic trough, and ascending faster and 
faster in the jar; the zinc now is acting by reason of its contact with 
the copper. 

Every step we are now taking brings us to a knowledge of new 
phenomena. That hydrogen which you now see coming off so 
abundantly does not come from the zinc, as it did before, but from 
the copper. Here is a jar containing a solution of copper. If I put a 
piece of this amalgamated zinc into it, and leave it there, it has 
scarcely any action; and here is a plate of platinum which I will 
immerse in the same solution, and might leave it there for hours, 
days, months, or even years, and no action would take place; but, 
by putting them both together, and allowing them to touch (Fig. 44), 
you see what a coating of copper there is immediately thrown down 
on the platinum. Why is this } The platinum has no power of itself 
to reduce that metal from that fluid, but it has, in some mysterious 
way, received this power by its contact with the metal zinc. Here, 
then, you see a strange transfer of chemical force from one metal to 
another; the chemical force from the zinc is transferred and made 
over to the platinum by the mere association of the two metals. I 
might take, instead of the platinum, a piece of copper or of silver, 
and it would have no action of its own on this solution, but the 
moment the zinc was introduced and touched the other metal, then 
the action would take place, and it would become covered with 
copper. Now is not this most wonderful and beautiful to see? We 
still have the identical chemical force of the particles of zinc acting, 
and yet, in some strange manner, we have power to make that 
chemical force, or something it produces, travel from one place to 
another; for we do make the chemical force travel from the zinc to 
the platinum by this very curious experiment of using the two 
metals in the same fluid in contact with each other. 



'](> FARADAY 

Let us now examine these phenomena a Uttle more closely. Here 
is a drawing (Fig. 45) in which I have represented a vessel con- 
taining the acid liquid and the slips of zinc and platinum or copper, 
and I have shown them touching each other outside by means of 
a wire coming from each of them (for it matters not whether they 
touch in the fluid or outside; by pieces of metal attached, they still, 
by that communication between them, have this power transferred 
from one to the other) . Now if, instead of only using one vessel, as 
I have shown there, I take another, and another, and put in zinc 





Fig. 44 



Fig. 45 



and platinum, zinc and platinum, zinc and platinum, and connect the 
platinum of one vessel with the zinc of another, the platinum of this 
vessel with the zinc of that, and so on, we should only be using a 
series of these vessels instead of one. This we have done in that 
arrangement which you see behind me. I am using what we call a 
Grove's voltaic battery, in which one metal is zinc and the other plat- 
inum; and I have as many as forty pairs of these plates all exercising 
their force at once in sending the whole amount of chemical power 
there evolved through these wires under the floor and up to these two 
rods coming through the table. We need do no more than just bring 
these two ends in contact, when the spark shows us what power is 
present; and what a strange thing it is to see that this force is brought 
away from the battery behind me, and carried along through these 
wires! I have here an apparatus (Fig. 46) which Sir Humphry Davy 
constructed many years ago, in order to see whether this power from 
the voltaic battery caused bodies to attract each other in the same 
manner as the ordinary electricity did. He made it in order to 
experiment with his large voltaic battery, which was the most power- 




THE CORRELATION OF THE PHYSICAL FORCES 77 

ful then in existence. You see there are in this glass jar two leaves of 

gold, which I can cause to move to and fro by this rack-work. I will 

connect each of these gold leaves with separate ends of this battery, 

and if I have a sufficient number of plates in the battery, I shall 

be able to show you that there will be some attraction between those 

leaves even before they come in contact; if I bring them sufficiently 

near when they are in communication with the 

ends of the battery, they will be drawn gently 

together; and you will know when this takes 

place, because the power will cause the gold 

leaves to burn away, which they could only do 

when they touched each other. Now I am 

going to cause these two leaves of gold to 

approach gradually, and I have no doubt that 

some of you will see that they approach before 

they burn, and those who are too far off to see 

them approach will see by their burning that "^^"^ 

they have come together. Now they are attract- ' '' 

ing each other, long before the connection is complete, and there 

they go! burnt up in that brilliant flash, so strong is the force. You 

thus see, from the attractive force at the two ends of this battery, that 

these are really and truly electrical phenomena. 

Now let us consider what is this spark. I take these two ends 
and bring them together, and there I get this glorious spark like the 
sunlight in the heavens above us. What is this ? It is the same thing 
which you saw when I discharged the large electrical machine, when 
you saw one single bright flash; it is the same thing, only continued, 
because here we have a more effective arrangement. Instead of hav- 
ing a machine which we are obliged to turn for a long time together, 
we have here a chemical power which sends forth the spark; and it 
is wonderful and beautiful to see how this spark is carried about 
through these wires. I want you to perceive, if possible, that this 
very spark and the heat it produces (for there is heat) is neither 
more nor less than the chemical force of the zinc — its very force 
carried along wires and conveyed to this place. I am about to take a 
portion of the zinc and burn it in oxygen gas for the sake of show- 
ing you the kind of light produced by the actual combustion in 



yS FARADAY 

oxygen gas of some of this metal. [A tassel of zinc-foil was ignited 
at a spirit lamp and introduced into a jar of oxygen, when it burnt 
with a brilliant light.] That shows you what the affinity is when we 
come to consider it in its energy and power. And the zinc is being 
burned in the battery behind me at a much more rapid rate than you 
see in that jar, because the zinc is there dissolving and burning, 
and produces here this great electric light. That very same power 
which in that jar you saw evolved from the actual combustion of 
the zinc in oxygen, is carried along these wires and made evident 
here; and you may, if you please, consider that the zinc is burning 
in those cells, and that this is the light of that burning [bringing 
the two poles in contact and showing the electric light]; and we 
might so arrange our apparatus as to show that the amounts of power 
evolved in either case are identical. Having thus obtained power 
over the chemical force, how wonderfully we are able to convey it 
from place to place! When we use gunpowder for explosive pur- 
poses, we can send into the mine chemical 
affinity by means of this electricity; not 
having provided fire beforehand, we can 
send it in at the moment we require it. 
Now here (Fig. 47) is a vessel containing 
two charcoal points, and I bring it forward 
as an illustration of the wonderful power 
of conveying this force from place to place. 
I have merely to connect these by means of 
wires to the opposite ends of the battery, 
and bring the points in contact. See what 
an exhibition of force we have! We have 
exhausted the air so that the charcoal can 
not burn, and therefore the light you see is 
really the burning of the zinc in the cells 
behind me; there is no disappearance of the 
carbon, although we have that glorious electric light; and the moment 
I cut off the connection it stops. Here is a better instance to enable 
some of you to see the certainty with which we can convey this force, 
where under ordinary circumstances, chemical affinity would not act. 
We may absolutely take these two charcoal poles down under water, 




Fig. 47 



THE CORRELATION OF THE PHYSICAL FORCES 79 

and get our electric light there. There they are in the water, and 
you observe, when I bring them into connection, we have the same 
light as we had in that glass vessel. 

Now besides this production of light, we have all the other effects 
and powers of burning zinc. I have a few wires here which are not 
combustible, and I am going to take one of them, a small platinum 
wire, and suspend it between these two rods which are connected 
with the battery, and when contact is made at the battery see what 




Fig. 48 

heat we get (Fig. 48). Is not that beautiful? . It is a complete bridge 
of power. There is metallic connection all the way round in this 
arrangement, and where I have inserted the platinum, which offers 
some resistance to the passage of the force, you see what an amount 
of heat is evolved; this is the heat which the zinc would give if burnt 
in oxygen; but, as it is being burnt in the voltaic battery, it is giving 
it out at this spot. I will now shorten this wire for the sake of show- 
ing you that, the shorter the obstructing wire is, the more and more 
intense is the heat, until at last our platinum is fused and falls down, 
breaking off the circuit. 

Here is another instance. I will take a piece of the metal silver, 
and place it on charcoal connected with one end of the battery, and 
lower the other charcoal pole on to it. See how brilliantly it burns! 
(Fig. 49.) Here is a piece of iron on the charcoal: see what a com- 
bustion is going on; and we might go on in this way, burning 
almost every thing we place between the poles. Now I want to 
show you that this power is still chemical affinity; that if we call the 
power which is evolved at this point heat, or electricity, or any other 
name referring to its source, or the way in which it travels, we still 



80 FARADAY 

shall find it to be chemical action. Here is a colored liquid which 
can show by its change of color the effects of chemical action; I will 
pour part of it into this glass, and you will find that these wires have 
a very strong action. I am not going to show you any effects of com- 




FiG. 49 

bustion or heat, but I will take these two platinum plates, and fasten 
one to the one pole and the other to the other end, and place them 
in this solution, and in a very short time you will see the blue color 
will be entirely destroyed. See, it is colorless now! I have merely 
brought the end of the wires into the solution of indigo, and the 
power of electricity has come through these wires and made itself 
evident by its chemical action. There is also another curious thing 
to be noticed now we are dealing with the chemistry of electricity, 
which is, that the chemical power which destroys the color is only 

due to the action on one side. I will 
pour some more of this sulphindigotic 
acid(") into a flat dish, and will then 
make a porous dike of sand separating 
the two portions of fluid into two parts 
(Fig. 50), and now we shall be able to 
see whether there is any difference in 
the two ends of the battery, and which 
it is that possesses this peculiar action. You see it is the one on my 
right hand which has the power of destroying the blue, for the 
portion on that side is thoroughly bleached, while nothing has 
apparently occurred on the other side. I say apparently, for you 

^'^Sulphindigotic acid. A mixture of one part of indigo and fifteen parts of con- 
centrated oil of vitriol. It is bleached on the side at which hydrogen gas is evolved 
in consequence of the liberated hydrogen withdrawing oxygen from the indigo, thereby 
forming a colorless deoxidized indigo. In making the experiment, only enough of the 
sulphindigotic acid must be added to give the water a decided blue color. 




Fig. 50 



THE CORRELATION OF THE PHYSICAL FORCES 8 1 

must not imagine that because you can not perceive any action none 
has taken place. 

Here we have another instance of chemical action. I take these 
platinum plates again and immerse them in this solution of copper, 
from which we formerly precipitated some of the metal, when the 
platinum and zinc were both put in it together. You see that these 
two platinum plates have no chemical action of any kind; they might 
remain in the solution as long as I liked, without having any power 
of themselves to reduce the copper; but the moment I bring the two 
poles of the battery in contact with them, the chemical action which 
is there transformed into electricity and carried along the wires again 
becomes chemical action at the two platinum poles, and now we 
shall have the power appearing on the left-hand side, and throwing 
down the copper in the metallic state on the platinum plate; and in 
this way I might give you many instances of the extraordinary way 
in which this chemical action or electricity may be carried about. 
That strange nugget of gold, of which there is a model in the other 
room, and which has an interest of its own in the natural history 
of gold, and which came from Ballarat, and was worth ^(^ 8,000 or 
^^9,000 when it was melted down last November, was brought 
together in the bowels of the earth, perhaps ages and ages ago, by 
some such power as this. And there is also another beautiful result 
dependent upon chemical affinity in that fine lead-tree(^^), the lead 
growing and growing by virtue of this power. The lead and the 
zinc are combined together in a little voltaic arrangement in a man- 
ner far more important than the powerful one you see here, because 
in nature these minute actions are going on forever, and are of 
great and wonderful importance in the precipitation of metals and 
formation of mineral veins, and so forth. These actions are not for 
a limited time, like my battery here, but they act forever in small 
degrees, accumulating more and more of the results. 

I have here given you all the illustrations that time will permit me 

^' Lead tree. To make a lead tree, pass a bundle of brass wires through the cork 
of a bottle, and fasten a plate of zinc round them just as they issue from the cork, 
so that the zinc may be in contact with every one of the wires. Make the wires to 
diverge so as to form a sort of cone, and, having filled the bottle quite full of a 
solution of sugar of lead, insert the wires and cork, and seal it down, so as to perfectly 
exclude the air. In a short time the metallic lead will begin to crystallize around the 
divergent wires, and form a beautiful object. 



82 FARADAY 

to show you of chemical affinity producing electricity, and electricity 
again becoming chemical affinity. Let that suffice for the present; 
and now let us go a little deeper into the subject of this chemical 
force, or this electricity — which shall I name first? — the one pro- 
ducing the other in a variety of ways. These forces are also won- 
derful in their power of producing another of the forces we have 
been considering, namely, that of magnetism; and you know that 
it is only of late years, and long since I was born, that the discovery 
of the relations of these two forces of electricity and chemical 
affinity to produce magnetism has become known. Philosophers 
had been suspecting this affinity for a long time, and had long had 
great hopes of success; for in the pursuit of science we first start 
with hopes and expectations; these we realize and estabUsh, never 
again to be lost, and upon them we found new expectations of farther 
discoveries, and so go on pursuing, realizing, establishing, and 
founding new hopes again and again. 

Now observe this: here is a piece of wire which I am about to 
make into a bridge of force, that is to say, a communicator between 



4W) *-+ 






m-* fl 


f 
















rBi r— 


^J 


u>= 


ZnX 


i 



Fig. 51 

the two ends of the battery. It is copper wire only, and is therefore 
not magnetic of itself. We will examine this wire with our magnetic 
needle (Fig. 51), and, though connected with one extreme end of 
the battery, you see that before the circuit is completed it has no 
power over the magnet. But observe it when I make contact; watch 
the needle; see how it is swung round; and notice how indifferent 
it becomes if I break contact again; so, you see, we have this wire 
evidently affecting the magnetic needle under these circumstances. 
Let me show you that a little more strongly. I have here a quantity 
of wire which has been wound into a spiral, and this will affect the 



THE CORRELATION OF THE PHYSICAL FORCES 83 

magnetic needle in a very curious manner, because, owing to its 
shape, it will act very like a real magnet. The copper spiral has 
no power over that magnetic needle at present; but if I cause the 
electric current to circulate through it, by bringing the two ends of 
the battery in contact with the ends of the wire which forms the 
spiral, what will happen? Why, one end of the needle is most 
powerfully drawn to it; and if I take the other end of the needle, 
it is repelled; so, you see, I have produced exactly the same phe- 
nomena as I had with the bar magnet, one end attracting and the 
other repelling. Is not this, then, curious to see, that we can con- 
struct a magnet of copper? Furthermore, if I take an iron bar, and 
put it inside the coil, so long as there is no electric current circulating 
round, it has no attraction, as you will observe if I bring a little iron 
filings or nails near the iron. But now, if I make contact with the 
battery, they are attracted at once. It becomes at once a powerful 
magnet, so much so that I should not wonder if these magnetic 
needles on different parts of the table pointed to it. And I will show 
you, by another experiment, what an attraction it has. This piece 

and that piece of iron, 
and many other pieces, 
are now strongly at- 
tracted (Fig. 52); but, 
as soon as I break con- 
tact, the power is all 
gone, and they fall. 

rWf What, then, can be a better or a stronger proof than this 
T of the relation of the powers of magnetism and electricity ? 
^ Again: here is a little piece of iron which is not mag- 
netized. It will not, at present, take up any one of these 
Fig. 52 nails; but I will take a piece of wire and coil it round the 
iron (the wire being covered with cotton in every part it 
does not touch the iron), so that the current must go round in this 
spiral coil; I am, in fact, preparing an electro-magnet (we are 
obliged to use such terms to express our meaning, because it is a 
magnet made by electricity — because we produce by the force of 
electricity a magnet of far greater power than a permanent steel 
one). It is now completed, and I will repeat the experiment which 




84 FARADAY 

you saw the other day, of building up a bridge of iron nails: the con- 
tact is now made and the current is going through; it is now a 
powerful magnet; here are the iron nails which we had the other 
day, and now I have brought this magnet near them they are 

clinging so hard that I can scarcely move 
them with my hand (Fig. 53) . But when 
the contact is broken, see how they fall. 
What can show you better than such an 
experiment as this the magnetic attrac- 
tion with which we have endowed these 
portions of iron? Here, again, is a fine 
illustration of this strong power of mag- 
netism. It is a magnet of the same sort as 
the one you have just seen. I am about 
to make the current of electricity pass 
through the wires which are round this 
iron for the purpose of showing you what 
powerful effects we get. Here are the poles of the magnet; and let us 
place on one of them this long bar of iron. You see, as soon as contact 
is made, how it rises in position (Fig. 54) ; and if I take such a piece 
as this cylinder, and place it on, woe be to me if I get my finger 




Fig. 53 




between; I can roll it over, but if I try to pull it off, I might lift up 
the whole magnet, but I have no power to overcome the magnetic 
power which is here evident. I might give you an infinity of illus- 
trations of this high magnetic power. There is that long bar of 
iron held out, and I have no doubt that if I were to examine the 



THE CORRELATION OF THE PHYSICAL FORCES 85 

Other end I should find that it was a magnet. See what power it 
must have to support not only these nails, but all those lumps of iron 
hanging on to the end. What, then, can surpass these evidences o£ 
the change of chemical force into electricity, and electricity into 
magnetism? I might show you many other experiments whereby I 
could obtain electricity and chemical action, heat and light from a 
magnet, but what more need I show you to prove the universal 
correlation of the physical forces of matter, and their mutual con- 
version one into another.? 

And now let us give place as juveniles to the respect we owe to 
our elders, and for a time let me address myself to those of our 
seniors who have honored me with their presence during these lec- 
tures. I wish to claim this moment for the purpose of tendering our 
thanks to them, and my thanks to you all for the way in which you 
have borne the inconvenience that I at first subjected you to. I hope 
that the insight which you have here gained into some of the laws 
by which the universe is governed, may be the occasion of some 
among you turning your attention to these subjects; for what study 
is there more fitted to the mind of man than that of the physical 
sciences ? And what is there more capable of giving him an insight 
into the actions of those laws, a knowledge of which gives interest to 
the most trifling phenomenon of nature, and makes the observing 
student find 

"Tongues in trees, books in the running brooks, 
Sermons in stones, and good in every thing?" 



THE CHEMICAL HISTORY 
OF A CANDLE 

A COURSE OF LECTURES DELIVERED BEFORE A 

JUVENILE AUDIENCE AT THE ROYAL 

INSTITUTION 

LECTURE I 

A CANDLE: THE FLAME— ITS SOURCES— STRUCTURE- 
MOBILITY— BRIGHTNESS 

I PURPOSE, in return for the honor you do us by coming to see 
what are our proceedings here, to bring before you, in the course 
of these lectures, the Chemical History of a Candle. I have 
taken this subject on a former occasion, and, were it left to my own 
will, I should prefer to repeat it almost every year, so abundant is 
the interest that attaches itself to the subject, so wonderful are the 
varieties of outlet which it oflfers into the various departments of 
philosophy. There is not a law under which any part of this uni- 
verse is governed which does not come into play and is touched upon 
in these phenomena. There is no better, there is no more open door 
by which you can enter into the study of natural philosophy than by 
considering the physical phenomena of a candle. I trust, therefore, I 
shall not disappoint you in choosing this for my subject rather than 
any newer topic, which could not be better, were it even so good. 
And, before proceeding, let me say this also: that, though our 
subject be so great, and our intention that of treating it honestly, 
seriously, and philosophically, yet I mean to pass away from all those 
who are seniors among us. I claim the privilege of speaking to juve- 
niles as a juvenile myself. I have done so on former occasions, and, 
if you please, I shall do so again. And, though I stand here with the 
knowledge of having the words I utter given to the world, yet that 

86 



CHEMICAL HISTORY OF A CANDLE 87 

shall not deter me from speaking in the same familiar way to those 
whom I esteem nearest to me on this occasion. 

And now, my boys and girls, I must first tell you of what candles 
are made. Some are great curiosities. I have here some bits of tim- 
ber, branches of trees particularly famous for their burning. And 
here you see a piece of that very curious substance, taken out of some 
of the bogs in Ireland, called candle-wood; a hard, strong, excellent 
wood, evidently fitted for good work as a register of force, and yet, 
withal, burning so well that where it is found they make splinters 
of it, and torches, since it burns like a candle, and gives a very good 
light indeed. And in this wood we have one of the most beautiful 
illustrations of the general nature of a candle that I can possibly give. 
The fuel provided, the means of bringing that fuel to the place of 
chemical action, the regular and gradual supply of air to that place 
of action — heat and light — all produced by a little piece of wood of 
this kind, forming, in fact, a natural candle. 

But we must speak of candles as they are in commerce. Here are 
a couple of candles commonly called dips. They are made of lengths 
of cotton cut off, hung up by a loop, dipped into melted tallow, 
taken out again and cooled, then redipped, until there is an accumu- 
lation of tallow round the cotton. In order that you may have an 
idea of the various characters of these candles, you see these which 
I hold in my hand — they are very small and very curious. They are, 
or were, the candles used by the miners in coal mines. In olden 
times the miner had to find his own candles, and it was supposed 
that a small candle would not so soon set fire to the fire-damp in the 
coal mines as a large one; and for that reason, as well as for economy's 
sake, he had candles made of this sort — 20, 30, 40, or 60 to the pound. 
They have been replaced since then by the steel-mill, and then by the 
Davy lamp, and other safety lamps of various kinds. I have here a 
candle that was taken out of the Royal Georgei^), it is said, by 
Colonel Pasley. It has been sunk in the sea for many years, subject 
to the action of salt water. It shows you how well candles may be 
preserved; for, though it is cracked about and broken a great deal, 

'The Royal George sunk at Spithead on the 29th of August, 1782. Colonel Pasley 
commenced operations for the removal of the wreck by the explosion of gunpowder, 
in August, 1839. The candle which Professor Faraday exhibited must therefore have 
been exposed to the action of salt water for upward of fifty-seven years. 



88 FARADAY 

yet when lighted it goes on burning regularly, and the tallow 
resumes its natural condition as soon as it is fused. 

Mr. Field, of Lambeth, has supplied me abundantly with beauti- 
ful illustrations of the candle and its materials; I shall therefore now 
refer to them. And, first, there is the suet — the fat of the ox — 
Russian tallow, I believe, employed in the manufacture of these dips, 
which Gay-Lussac, or some one who intrusted him with his knowl- 
edge, converted into that beautiful substance, stearin, which you see 
lying beside it. A candle, you know, is not now a greasy thing like 
an ordinary tallow candle, but a clean thing, and you may almost 
scrape off and pulverize the drops which fall from it without soiling 
any thing. This is the process he adopted(^) : The fat or tallow is 
first boiled with quick-lime, and made into a soap, and then the soap 
is decomposed by sulphuric acid, which takes away the lime, and 
leaves the fat rearranged as stearic acid, while a quantity of glycerin 
is produced at the same time. Glycerin — absolutely a sugar, or a 
substance similar to sugar — comes out of the tallow in this chemical 
change. The oil is then pressed out of it; and you see here this series 
of pressed cakes, showing how beautifully the impurities are carried 
out by the oily part as the pressure goes on increasing, and at last you 
have left that substance, which is melted, and cast into candles as 
here represented. The candle I have in my hand is a stearin candle, 
made of stearin from tallow in the way I have told you. Then here 
is a sperm candle, which comes from the purified oil of the sper- 
maceti whale. Here, also, are yellow beeswax and refined beeswax, 
from which candles are made. Here, too, is that curious substance 
called parafEne, and some paraffine candles, made of parafEne 
obtained from the bogs of Ireland. I have here also a substance 
brought from Japan since we have forced an entrance into 

2 The fat or tallow consists of a chemical combination of fatty acids with glycerin. 
The lime unites with the palmitic, oleic, and stearic acids, and separates the glycerin. 
After washing, the insoluble lime soap is decomposed with hot dilute sulphuric acid. 
The melted fatty acids thus rise as an oil to the surface, when they are decanted. 
They are again washed and cast into thin plates, which, when cold, are placed between 
layers of cocoanut matting and submitted to intense hydraulic pressure. In this way 
the soft oleic acid is squeezed out, while the hard palmitic and stearic acids remain. 
These are farther purified by pressure at a higher temperature and washing in warm 
dilute sulphuric acid, when they are ready to be made into candles. These acids 
are harder and whiter than the fats from which they were obtained, while at the 
same time they are cleaner and more combustible. 



CHEMICAL HISTORY OF A CANDLE 89 

that out-of-the-way place — a sort of wax which a kind friend has 
sent me, and which forms a new material for the manufacture of 
candles. 

And how are these candles made? I have told you about dips, 
and I will show you how moulds are made. Let us imagine any of 
these candles to be made of materials which can be cast. "Cast!" 
you say. "Why, a candle is a thing that melts, and surely if you can 
melt it you can cast it." Not so. It is wonderful, in the progress of 
manufacture, and in the consideration of the means best fitted to 
produce the required result, how things turn up which one would 
not expect beforehand. Candles can not always be cast. A wax 
candle can never be cast. It is made by a particular process which I 
can illustrate in a minute or two, but I must not spend much time on 
it. Wax is a thing which, burning so well, and melting so easily in 
a candle, can not be cast. However, let us take a material that can 
be cast. Here is a frame, with a number of moulds fastened in it. 
The first thing to be done is to put a wick through them. Here is 
one — a plaited wick, which does not require snuffing(^) — supported 
by a little wire. It goes to the bottom, where it is pegged in — the 
little peg holding the cotton tight, and stopping the aperture so that 
nothing fluid shall run out. At the upper part there is a little bar 
placed across, which stretches the cotton and holds it in the mould. 
The tallow is then melted, and the moulds are filled. After a certain 
time, when the moulds are cool, the excess of tallow is poured off 
at one corner, and then cleaned off altogether, and the ends of the 
wick cut away. The candles alone then remain in the mould, and 
you have only to upset them, as I am doing, when out they tumble, 
for the candles are made in the form of cones, being narrower at 
the top than at the bottom; so that, what with their form and their 
own shrinking, they only need a little shaking, and out they fall. 
In the same way are made these candles of stearin and of paraffine. 
It is a curious thing to see how wax candles are made. A lot of 
cottons are hung upon frames, as you see here, and covered with 
metal tags at the ends to keep the wax from covering the cotton in 
those places. These are carried to a heater, where the wax is melted. 

' A Htde borax or phosphorus salt is sometimes added in order to make the ash 
fusible. 



^„ '*" — — - 

As you see, the frames can turn round; and, as they turn, a man 
takes a vessel of wax and pours it first down one, and then the next, 
and the next, and so on. When he has gone once round, if it isi 
sufficiently cool, he gives the first a second coat, and so on until they 
are all of the required thickness. When they have been thus clothed, 
or fed, or made up to that thickness, they are taken off and placed 
elsewhere. I have here, by the kindness of Mr. Field, several speci- 
mens of these candles. Here is one only half finished. They are then 
taken down and well rolled upon a fine stone slab, and the conical 
top is moulded by properly shaped tubes, and the bottoms cut off 
and trimmed. This is done so beautifully that they can make 
candles in this way weighing exactly four or six to the pound, or 
any number they please. 

We must not, however, take up more time about the mere manu- 
facture, but go a little farther into the matter. I have not yet 
referred you to luxuries in candles (for there is such a thing as luxury 
in candles). See how beautifully these are colored; you see here 
mauve, magenta, and all the chemical colors recendy introduced, 
applied to candles. You observe, also, different forms employed. 
Here is a fluted pillar most beautifully shaped; and I have also here 
some candles sent me by Mr. Pearsall, which are ornamented with 
designs upon them, so that, as they burn, you have, as it were, a 
glowing sun above, and bouquet of flowers beneath. All, however, 
that is fine and beautiful is not useful. These fluted candles, pretty 
as they are, are bad candles; they are bad because of their external 
shape. Nevertheless, I show you these specimens, sent to me from 
kind friends on all sides, that you may see what is done and what 
may be done in this or that direction; although, as I have said, when 
we come to these refinements, we are obliged to sacrifice a little in 
utility. 

Now as to the light of the candle. We will light one or two, and 
set them at work in the performance of their proper functions. You 
observe a candle is a very different thing from a lamp. With a lamp 
you take a little oil, fill your vessel, put in a little moss or some cotton 
prepared by artificial means, and then light the top of the wick. 
When the flame runs down the cotton to the oil, it gets extinguished, 
but it goes on burning in the part above. Now I have no doubt you 
will ask how it is that the oil which will not burn of itself gets up to 



CHEMICAL HISTORY OF A CANDLE pi 

the top of the cotton, where it will burn. We shall presently examine 
that; but there is a much more wonderful thing about the burning of 
a candle than this. You have here a solid substance with no vessel to 
contain it; and how is it that this solid substance can get up to the 
place where the flame is ? How is it that this solid gets there, it not 
being a fluid? or, when it is made a fluid, then how is it that it 
keeps together? This is a wonderful thing about a candle. 

We have here a good deal of wind, which will help us in some of 
our illustrations, but tease us in others; for the sake, therefore, of a 
little regularity, and to simplify the matter, I shall make a quiet 
flame, for who can study a subject when there are difficulties in 
the way not belonging to it? Here is a clever invention of some 
costermonger or street-stander in the market-place for the shading 
of their candles on Saturday nights, when they are selling their 
greens, or potatoes, or fish. I have very often admired it. They put a 
lamp-glass round the candle, supported on a kind of gallery, which 
clasps it, and it can be slipped up and down as required. By the use 
of this lamp-glass, employed in the same way, you have a steady 
flame, which you can look at, and carefully examine, as I hope you 
will do, at home. 

You see, then, in the first instance, that a beautiful cup is formed. 
As the air comes to the candle, it moves upward by the force of the 
current which the heat of the candle produces, and it so cools all the 
sides of the wax, tallow, or fuel as to keep the edge much cooler than 
the part within; the part within melts by the flame that runs down 
the wick as far as it can go before it is extinguished, but the part on 
the outside does not melt. If I made a current in one direction, my 
cup would be lop-sided, and the fluid would consequently run over; 
for the same force of gravity which holds worlds together holds this 
fluid in a horizontal position, and if the cup be not horizontal, of 
course the fluid will run away in guttering. You see, therefore, that 
the cup is formed by this beautifully regular ascending current of 
air playing upon all sides, which keeps the exterior of the candle 
cool. No fuel would serve for a candle which has not the property of 
giving this cup, except such fuel as the Irish bogwood, where the 
material itself is like a sponge and holds its own fuel. You see now 
why you would have had such a bad result if you were to burn these 
beautiful candles that I have shown you, which are irregular, inter- 



92 FARADAY 

mittent in their shape, and can not, therefore, have that nicely-formed 
edge to the cup which is the great beauty in a candle. I hope you 
will now see that the perfection of a process — that is, its utility — is 
the better point of beauty about it. It is not the best looking thing, 
but the best acting thing, which is the most advantageous to us. This 
good-looking candle is a bad-burning one. There will be a guttering 
round about it because of the irregularity of the stream of air and the 
badness of the cup which is formed thereby. You may see some 
pretty examples (and I trust you will notice these instances) of the 
action of the ascending current when you have a little gutter run 
down the side of a candle, making it thicker there than it is else- 
where. As the candle goes on burning, that keeps its place and forms 
a little pillar sticking up by the side, because, as it rises higher above 
the rest of the wax or fuel, the air gets better round it, and it is more 
cooled and better able to resist the action of the heat at a little dis- 
tance. Now the greatest mistakes and faults with regard to candles, 
as in many other things, often bring with them instruction which we 
should not receive if they had not occurred. We come here to be 
philosophers, and I hope you will always remember that whenever a 
result happens, especially if it be new, you should say, "What is the 
cause? Why does it occur.''" and you will, in the course of time, find 
out the reason. 

Then there is another point about these candles which will answer 
a question — that is, as to the way in which this fluid gets out of the 
cup, up the wick, and into the place of combustion. You know that 
the flames on these burning wicks in candles made of beeswax, 
stearin, or spermaceti, do not run down to the wax or other matter, 
and melt it all away, but keep to their own right place. They are 
fenced off from the fluid below, and do not encroach on the cup at 
the sides. I can not imagine a more beautiful example than the con- 
dition of adjustment under which a candle makes one part subserve 
to the other to the very end of its action. A combustible thing like 
that, burning away gradually, never being intruded upon by the 
flame, is a very beautiful sight, especially when you come to learn 
what a vigorous thing flame is — what power it has of destroying the 
wax itself when it gets hold of it, and of disturbing its proper form if 
it come only too near. 



CHEMICAL HISTORY OF A CANDLE 93 

But how does the flame get hold of the fuel ? There is a beautiful 
point about that — capillary attraction.^) "Capillary attraction!" you 
say — "the attraction of hairs." Well, never mind the name; it was 
given in old times, before we had a good understanding of what the 
real power was. It is by what is called capillary attraction that the 
fuel is conveyed to the part where combustion goes on, and is de- 
posited there, not in a careless way, but very beautifully in the very 
midst of the centre of action, which takes place around it. Now 
I am going to give you one or two instances of capillary attraction. It 
is that kind of action or attraction which makes two things that do 
not dissolve in each other still hold together. When you wash your 
hands, you wet them thoroughly; you take a little soap to make the 
adhesion better, and you find your hands remain wet. This is by that 
kind of attraction of which I am about to speak. And, what is more, 
if your hands are not soiled (as they almost always are by the usages 
of life), if you put your finger into a little warm water, the water 
will creep a little way up the finger, though you may not stop to 
examine it. I have here a substance which is rather porous — a column 
of salt — and I will pour into the plate 
at the bottom, not water, as it appears, 
but a saturated solution of salt which 
can not absorb more, so that the 
action which you see will not be due 
to its dissolving any thing. We may 
consider the plate to be the candle, 
and the salt the wick, and this solu- 
tion the melted tallow. (I have 
colored the fluid, that you may see the 
action better.) You observe that, now 
I pour in the fluid, it rises and gradually creeps up the salt higher and 
higher (Fig. 55) ; and provided the column does not tumble over, it 
will go to the top. If this blue solution were combustible, and we were 
to place a wick at the top of the salt, it would burn as it entered into 

^ Capillary attraction or repulsion is the cause which determines the ascent or 
descent of a fluid in a capillary tube. If a piece of thermometer tubing, open at each 
end, be plunged into water, the latter will instantly rise in the tube considerably above 
its external level. If, on the other hand, the tube be plunged into mercury, a repulsion 
instead of attraction will be exhibited, and the level of the mercury will be lower in 
the tube than it is outside. 




Fig. 55 



94 FARADAY 

the wick. It is a most curious thing to see this kind of action taking 
place, and to observe how singular some of the circumstances are 
about it. When you wash your hands, you take a towel to wipe ofl 
the water; and it is by that kind of wetting, or that kind of attraction 
which makes the towel become wet with water, that the wick is 
made wet with the taliow. I have known some careless boys and girls 
(indeed, I have known it happen to careful people as well) who, 
having washed their hands and wiped them with a towel, have 
thrown the towel over the side of the basin, and before long it has 
drawn all the water out of the basin and conveyed it to the floor, 
because it happened to be thrown over the side in such a way as to 
serve the purpose of a siphon. C") That you may the better see the 
way in which the substances act one upon another, I have here a 
vessel made of wire gauze filled with water, and you may compare it 
in its action to the cotton in one respect, or to a piece of calico in the 
other. In fact, wicks are sometimes made of a kind of wire gauze. 
You will observe that this vessel is a porous thing; for if I pour a little 
water on to the top, it will run out at the bottom. You would be 
puzzled for a good while if I asked you what the state of this vessel 
is, what is inside it, and why it is there? The vessel is full of water, 
and yet you see the water goes in and runs out as if it were empty. 
In order to prove this to you, I have only to empty it. The reason is 
this: the wire, being once wetted, remains wet; the meshes are so 
small that the fluid is attracted so strongly from the one side to 
the other, as to remain in the vessel, although it is porous. In like 
manner, the particles of melted tallow ascend the cotton and 
get to the top; other particles then follow by their mutual attrac- 
tion for each other, and as they reach the flame they are gradually 
burned. 

Here is another application of the same principle. You see this bit 
of cane. I have seen boys about the streets, who are very anxious to 
appear like men, take a piece of cane, and light it, and smoke it, as 
an imitation of a cigar. They are enabled to do so by the permeability 

^ The late Duke of Sussex was, we believe, the first to show that a prawn might 
be washed upon this principle. If the tail, after pulling off the fan part, be placed in a 
tumbler of water, and the head be allowed to hang over the outside, the water will 
be sucked up the tail by capillary attraction, and will continue to run out through 
the head until the water in the glass has sunk so low that the tail ceases to dip into it. 




CHEMICAL HISTORY OF A CANDLE 95 

of the cane in one direction, and by its capillarity. If I place this piece 
of cane on a plate containing some camphene (which is very much 
like paraiSne in its general character), exactly in the same manner as 
the blue fluid rose through the salt will this fluid rise through the 
piece of cane. There being no pores at the side, the fluid can not go 

in that direction, but must pass through 
i its length. Already the fluid is at the top 
of the cane; now I can light it and make 
it serve as a candle. The fluid has risen 
by the capillary attraction of the piece 
of cane, just as it does through the cotton in the candle. 
Now the only reason why the candle does not burn 
all down the side of the wick is that the melted tallow 
extinguishes the flame. You know that a candle, if 
turned upside down, so as to allow the fuel to run 
upon the wick, will be put out. The reason is, that the 
flame has not had time to make the fuel hot enough 
to burn, as it does above, where it is carried in small 
quantities into the wick, and has all the effect of the 
heat exercised upon it. 
There is another condition which you must learn as regards the 
candle, without which you would not be able fully to understand 
the philosophy of it, and that is the vaporous condition of the fuel. 
In order that you may understand that, let me show you a very pretty 
but very commonplace experiment. If you blow a candle out cleverly, 
you will see the vapor rise from it. You have, I know, often smelt 
the vapor of a blown-out candle, and a very bad smell it is; but if 
you blow it out cleverly you will be able to see pretty well the vapor 
into which this solid matter is transformed. I will blow out one of 
these candles in such a way as not to disturb the air around it by the 
continuing action of my breath; and now, if I hold a lighted taper 
two or three inches from the wick, you will observe a train of fire 
going through the air till it reaches the candle (Fig. 56) . I am obliged 
to be quick and ready, because if I allow the vapor time to cool, it 
becomes condensed into a liquid or solid, or the stream of com- 
bustible matter gets disturbed. 
Now as to the shape or form of the flame. It concerns us much to 




Fig. 56 



96 FARADAY 

know about the condition which the matter of the candle finally 
assumes at the top of the wick, where you have such beauty and 
brightness as nothing but combustion or flame can produce. You 
have the glittering beauty of gold and silver, and the still higher 
lustre of jewels like the ruby and diamond; but none of these rival 
the brilliancy and beauty of flame. What diamond can shine like 
flame? It owes its lustre at nighttime to the very flame 
A shining upon it. The flame shines in darkness, but the light 
which the diamond has is as nothing until the flame shines 
upon it, when it is brilliant again. The candle alone shines 
by itself and for itself, or for those who have arranged the 
materials. Now let us look a little at the form of the flame 
as you see it under the glass shade. It is steady and equal, 
and its general form is that which is represented in the dia- 
gram (Fig. 57), varying with atmospheric disturbances, and 
also varying according to the size of the candle. It is a 
bright oblong, brighter at the top than toward the bottom, 
with the wick in the middle, and, besides the wick in the 
middle, certain darker parts towards the bottom, where the 
ignition is not so.perfect as in the part above. I have a draw- 
ing here, sketched many years ago by Hooker, when he 
made his investigations. It is the drawing of the flame of a lamp, but 
it will apply to the flame of a candle. The cup of the candle is the ves- 
sel or lamp; the melted spermaceti is the oil; and the wick is common 
to both. Upon that he sets this little flame, and then he represents 
what is true, a certain quantity of matter rising about it which you do 
not see, and which, if you have not been here before, or are not famil- 
iar with the subject, you will not know of. He has here represented 
the parts of the surrounding atmosphere that are very essential to the 
flame, and that are always present with it. There is a current formed, 
which draws the flame out; for the flame which you see is really 
drawn out by the current, and drawn upward to a great height, just as 
Hooker has here shown you by that prolongation of the current in the 
diagram. You may see this by taking a lighted candle, and putting it 
in the sun so as to get its shadow thrown on a piece of paper. How re- 
markable it is that that thing which is light enough to produce shad- 
ows of other objects can be made to throw its own shadow on a piece 



Fig. 57 



Ili!!!!!!!! 

,,l|l,|].!,i!,l„ 



CHEMICAL HISTORY OF A CANDLE 97 

of white paper or card, so that you can actually see streaming round 
the flame something which is not part of the flame, but is ascending 
and drawing the flame upward. Now I am going to imitate the sun- 
hght by applying the voltaic battery to the electric lamp. You now see 
our sun and its great luminosity; and by placing a candle 
between it and the screen, we get the shadow of the 
flame. You observe the shadow of the candle and of the 
wick; then there is a darkish part, as represented in the 
diagram (Fig. 58), and then a part which is more dis- 
tinct. Curiously enough, however, what we see in the 
shadow as the darkest part of the flame is, in reality, the 
brightest part; and here you see streaming upward the 
ascending current of hot air, as shown by Hooker, which 
draws out the flame, supplies it with air, and cools the 
sides of the cup of melted fuel. 

I can give you here a little farther illustration, for the 
purpose of showing you how flame goes up or down 
according to the current. I have here a flame — it is 
not a candle flame — but you can, no doubt, by this 
time generalize enough to be able to compare one thing with 
another. What I am about to do is to change the ascending current 
that takes the flame upward into a descending current. This I can 
easily do by the little apparatus you see before me. The flame, as I 
have said, is not a candle flame, but it is produced by alcohol, so 
that it shall not smoke too much. I will also color the flame with 
another substance ("), so that you may trace its course; for, with the 
spirit alone, you could hardly see well enough to have the opportunity 
of tracing its direction. By lighting this spirit of wine we have then 
a flame produced, and you observe that when held in the air it 
naturally goes upward. You understand now, easily enough, why 
flames go up under ordinary circumstances: it is because of the 
draught of air by which the combustion is formed. But now, by 
blowing the flame down, you see I am enabled to make it go down- 
ward into this little chimney, the direction of the current being 
changed (Fig. 59). Before we have concluded this course of lectures 




Fig. 58 



flame. 



The alcohol had chloride of copper dissolved in it: this produces a beautiful green 
le. 



98 FARADAY 

we shall show you a lamp in which the flame goes up and the smoke 
goes down, or the flame goes down and the smoke goes up. You see, 
then, that we have the power in this way of varying the flame in 
different directions. 

There are now some other points that I must bring before you. 
Many of the flames you see here vary very much in their shape by 

the currents of air blowing around them 
in different directions; but we can, if we 
like, make flames so that they will look 
like fixtures, and we can photograph them 
-indeed, we have to photograph them — 
so that they become fixed to us, if we wish 
to find out every thing concerning them. 
That, however, is not the only thing I 
wish to mention. If I take a flame suffi- 
ciently large, it does not keep that homo- 
geneous, that uniform condition of shape, 
but it breaks out with a power of life 
which is quite wonderful. I am about to 
use another kind of fuel, but one which is truly and fairly a repre- 
sentative of the wax or tallow of a candle. I have here a large ball of 
cotton, which will serve as a wick. And, now that I have immersed 
it in spirit and applied a light to it, in what way does it differ from 
an ordinary candle? Why, it differs very much in one respect, that 
we have a vivacity and power about it, a beauty and a life entirely 
different from the light presented by a candle. You see those fine 
tongues of flame rising up. You have the same general disposition of 
the mass of the flame from below upward; but, in addition to that, 
you have this remarkable breaking out into tongues which you do 
not perceive in the case of a candle. Now, why is this? I must 
explain it to you, because, when you understand that perfectly, you 
will be able to follow me better in what I have to say hereafter. I 
suppose some here will have made for themselves the experiment I 
am going to show you. Am I right in supposing that any body here 
has played at snapdragon ? I do not know a more beautiful illustra- 
tion of the philosophy of flame, as to a certain part of its history, than 
the game of snapdragon. First, here is the dish; and let me say, that 




CHEMICAL HISTORY OF A CANDLE 99 

when you play snapdragon properly you ought to have the dish well 
warmed; you ought also to have warm plums, and warm brandy, 
which, however, I have not got. When you have put the spirit into 
the dish, you have the cup and the fuel; and are not the raisins acting 
like the wicks ? I now throw the plums into the dish, and light the 
spirit, and you see those beautiful tongues of flame that I refer to. 
You have the air creeping in over the edge of the dish forming these 




Fig. 60 



tongues. Why? Because, through the force of the current and the 
irregularity of the action of the flame, it can not flow in one uniform 
stream. The air flows in so irregularly that you have what would 
otherwise be a single image broken up into a variety of forms, and 
each of these little tongues has an independent existence of its own. 
Indeed, I might say, you have here a multitude of independent can- 
dles. You must not imagine, because you see these tongues all at 
once, that the flame is of this particular shape. A flame of that shape 
is never so at any one time. Never is a body of flame, like that 
which you just saw rising from the ball, of the shape it appears to 
you. It consists of a multitude of different shapes, succeeding each 
other so fast that the eye is only able to take cognizance of them all 
at once. In former times I purposely analyzed a flame of that general 
character, and the diagram shows you the different parts of which it 
is composed (Fig. 60). They do not occur all at once; it is only be- 
cause we see these shapes in such rapid succession that they seem to 
us to exist all at one time. 



100 FARADAY 



It is too bad that we have not got farther than my game of snap- 
dragon; but we must not, under any circumstances, keep you beyond 
your time. It will be a lesson to me in future to hold you more strictly 
to the philosophy of the thing than to take up your time so much 
with these illustrations. 



LECTURE II 

A CANDLE: BRIGHTNESS OF THE FLAME— AIR NECES- 
SARY FOR COMBUSTION— PRODUC- 
TION OF WATER 

WE were occupied the last time we met in considering the 
general character and arrangement as regards the fluid 
portion of a candle, and the way in which that fluid got 
into the place of combustion. You _ see, when we have a candle 
burning fairly in a regular, steady atmosphere, it will have a shape 
something like the one shown in the diagram, and will look pretty 
uniform, although very curious in its character. And now I have 
to ask your attention to the means by which we are enabled to ascer- 
tain what happens in any particular part of the flame; why it hap- 
pens; what it does in happening; and where, after all, the whole 
candle goes to; because, as you know very well, a candle being 
brought before us and burned, disappears, if burned properly, with- 
out the least trace of dirt in the candle stick; and this is a very 
curious circumstance. In order, then, to examine this candle care- 
fully, I have arranged certain apparatus, the use of which you will see 
as I go on. Here is a candle; I am about to put the end of this glass 
tube into the middle of the flame — into that part which old Hooker 
has represented in the diagram as being rather dark, and which you 
can see at any time if you will look at a candle carefully, without 
blowing it about. We will examine this dark part first. 

Now I take this bent glass tube, and introduce one end into that 
part of the flame, and you see at once that something is coming 
from the flame, out at the other end of the tube; and if I put a flask 
there, and leave it for a little while, you will see that something from 
the middle part of the flame is gradually drawn out, and goes 
through the tube, and into that flask, and there behaves very differ- 
ently from what it does in the open air. It not only escapes from the 
end of the tube, but falls down to the bottom of the flask like a 



102 



FARADAY 



heavy substance, as indeed it is (Fig. 6i). We find that this is the 
wax of the candle made into a vaporous fluid — not a gas. (You 
must learn the diflFerence between a gas and a vapor: a gas remains 
permanent; a vapor is something that will condense.) If you blow 
out a candle, you perceive a very nasty smell, resulting from the con- 
densation of this vapor. That is very different from what you have 
outside the flame; and, in order to make that more clear to you, I am 
about to produce and set fire to a larger portion of this vapor; for 




Fig. 6i 

what we have in the small way in a candle, to understand thor- 
oughly, we must, as philosophers, produce in a larger way, if needful, 
that we may examine the different parts. And now Mr. Anderson 
will give me a source of heat, and I am about to show you what that 
vapor is. Here is some wax in a glass flask, and I am going to make 
it hot, as the inside of that candle flame is hot, and the matter about 
the wick is hot. [The lecturer placed some wax in a glass flask, and 
heated it over a lamp.] Now I dare say that is hot enough for me. 
You see that the wax I put in it has become fluid, and there is a 
little smoke coming from it. We shall very soon have the vapor 
rising up. I will make it still hotter, and now we get more of it, so 
that I can actually pour the vapor out of the flask into that basin, and 
set it on fire there. This, then, is exactly the same kind of vapor as 




CHEMICAL HISTORY OF A CANDLE IO3 

we have in the middle of the candle; and that you may be sure 
this is the case, let us try whether we have not got here, in this flask, 
a real combustible vapor out o£ the middle of the candle. [Taking 
the flask into which the tube from the candle proceeded, and intro- 
ducing a lighted taper (Fig. 62).] See how it burns. Now this is the 

vapor from the middle of the candle, 

|C7 produced by its own heat; and that is 

one of the first things you have to 
consider with respect to the progress of the wax in the 
course of its combustion, and as regards the changes it under- 
goes. I will arrange another tube carefully in the flame, and 
I should not wonder if we were able, by a little care, to get 
that vapor to pass through the tube to the other extremity, 
where we will light it, and obtain absolutely the flame of the 
candle at a place distant from it. Now, look at that. Is not 
that a very pretty experiment ? Talk about laying on gas — 
why, we can actually lay on a candle! And you see from 
Fig. 62 this that there are clearly two different kinds of action — 
one the production of the vapor, and the other the combus- 
tion of it — both of which take place in particular parts of the candle. 
I shall get no vapor from that part which is already burnt. If I 
raise the tube (Fig. 61) to the upper part of the flame, so soon as the 
vapor has been swept out what comes away wall be no longer com- 
bustible; it is already burned. How burned? Why, burned thus: 
In the middle of the flame, where the wick is, there is this com- 
bustible vapor; on the outside of the flame is the air which we shall 
find necessary for the burning of the candle; between the two, in- 
tense chemical action takes place, whereby the air and the fuel act 
upon each other, and at the very same time that we obtain light the 
vapor inside is destroyed. If you examine where the heat of a candle 
is, you will find it very curiously arranged. Suppose I take this can- 
dle, and hold a piece of paper close upon the flame, where is the heat 
of that flame? Do you not see that it is not in the inside? It is in a 
ring, exactly in the place where I told you the chemical action was; 
and even in my irregular mode of making the experiment, if there 
is not too much disturbance, there will always be a ring. This is a 
good experiment for you to make at home. Take a strip of paper. 



104 FARADAY 

have the air in the room quiet, and put the piece of paper right 
across the middle of the flame — (I must not talk while I make the 
experiment) — and you will find that it is burnt in two places, and 
that it is not burnt, or very little so, in the middle; and when you 
have tried the experiment once or twice, so as to make it nicely, you 
will be very interested to see where the heat is, and to find that it is 
where the air and the fuel come together. 

This is most important for us as we proceed with our subject. Air 
is absolutely necessary for combustion; and, what is more, I must 
have you understand that fresh air is necessary, or else we should be 
imperfect in our reasoning and our experiments. Here is a jar of 
air; I place it over a candle, and it burns very nicely in it at first, 
showing that what I have said about it is true; but there will soon 
be a change. See how the flame is drawing upward, presently fading, 
and at last going out. And going out, why ? Not because it wants 
air merely, for the jar is as full now as it was before; but it wants 
pure, fresh air. The jar is full of air, partly changed, partly not 
changed; but it does not contain sufficient of the fresh air which is 
necessary for the combustion of a candle. These are all points which 
we, as young chemists, have to gather up; and if we look a little 
more closely into this kind of action, we shall find certain steps of 
reasoning extremely interesting. For instance, here is the oil-lamp 
I showed you — an excellent lamp for our experiments — the old 
Argand lamp. I now make it like a candle [obstructing the passage 
of air into the centre of the flame]; there is the cotton; there is the 
oil rising up in it, and there is the conical flame. It burns poorly 
because there is a partial restraint of air. I have allowed no air to 
get to it save around the outside of the flame, and it does not burn 
well. I can not admit more air from the outside, because the wick is 
large; but if, as Argand did so cleverly, I open a passage to the middle 
of the flame, and so let air come in there, you will see how much 
more beautifully it burns. If I shut the air off, look how it smokes; 
and why ? We have now some very interesting points to study : we 
have the case of the combustion of a candle; we have the case of a 
candle being put out by the want of air; and we have now the case 
of imperfect combustion, and this is to us so interesting that I want 
you to understand it as thoroughly as you do the case of a candle 



CHEMICAL HISTORY OF A CANDLE IO5 

burning in its best possible manner. I will now make a great flame, 
because we need the largest possible illustrations. Here is a larger 
wick [burning turpentine on a ball of cotton]. All these things are 
the same as candles, after all. If we have larger wicks, we must have 
a larger supply of air, or we shall have less perfect combustion. Look, 
now, at this black substance going up into the atmosphere; there is 
a regular stream of it. I have provided means to carry off the im- 
perfectly burned part, lest it should annoy you. Look at the soots 
that fly off from the flame; see what an imperfect combustion it is, 
because it can not get enough air. What, then, is happening.? Why, 
certain things which are necessary to the combustion of a candle 
are absent, and very bad results are accordingly produced; but we 
see what happens to a candle when it is burnt in a pure and proper 
state of air. At the time when I showed you this charring by the 
ring of flame on the one side of the paper, I might have also shown 
you, by turning to the other side, that the burning of a candle pro- 
duces the same kind of soot — charcoal, or carbon. 

But, before I show that, let me explain to you, as it is quite neces- 
sary for our purpose, that, though I take a candle, and give you, as 
the general result, its combustion in the form of a flame, we must 
see whether combustion is always in this condition, or whether there 
are other conditions of flame; and we shall soon discover that there 
are, and that they are most important to us. I think, perhaps, the 
best illustration of such a point to us, as juveniles, is to show the 
result of strong contrast. Here is a little gunpowder. You know that 
gunpowder burns with flame; we may fairly call it flame. It con- 
tains carbon and other materials, which altogether cause it to burn 
with a flame. And here is some pulverized iron, or iron filings. Now 
I purpose burning these two things together. I have a little mortar 
in which I will mix them. (Before I go into these experiments, let 
me hope that none of you, by trying to repeat them for fun's sake, 
will do any harm. These things may all be very properly used if 
you take care, but without that much mischief will be done.) Well, 
then, here is a little gunpowder, which I put at the bottom of that 
little wooden vessel, and mix the iron filings up with it, my object 
being to make the gunpowder set fire to the filings and burn them 
in the air, and thereby show the difference between substances burn- 



I06 FARADAY 

ing with flame and not with flame. Here is the mixture; and when 
I set fire to it you must watch the combustion, and you will see that 
it is of two kinds. You will see the gunpowder burning with a flame 
and the filings thrown up. You will see them burning, too, but 
without the production of flame. They will each burn separately. 
[The lecturer then ignited the mixture.] There is the gunpowder, 
which burns with a flame, and there are the filings: they burn with 
a different kind of combustion. You see, then, these two great 
distinctions; and upon these differences depend all the utility and all 
the beauty of flame which we use for the purpose of giving out light. 
When we use oil, or gas, or candle for the purpose of illumination, 
their fitness all depends upon these different kinds of combustion. 

There are such curious conditions of flame that it requires some 
cleverness and nicety of discrimination to distinguish the kinds of 
combustion one from another. For instance, here is a powder which 
is very combustible, consisting, as you see, of separate little particles. 
It is called lycopodium,C) and each of these particles can produce a 
vapor, and produce its own flame; but, to see them burning, you 
would imagine it was all one flame. I will now set fire to a quan- 
tity, and you will see the effect. We saw a cloud of flame, apparently 
in one body; but that rushing noise [referring to the sound produced 
by the burning] was a proof that the combustion was not a con- 
tinuous or regular one. This is the lightning of the pantomimes, and 
a very good imitation. [The experiment was twice repeated by blow- 
ing lycopodium from a glass tube through a spirit flame.] This is 
not an example of combustion like that of the filings I have been 
speaking of, to which we must now return. 

Suppose I take a candle and examine that part of it which appears 
brightest to our eyes. Why, there I get these black particles, which 
already you have seen many times evolved from the flame, and which 
I am now about to evolve in a different way. I will take this candle 
and clear away the gutterage, which occurs by reason of the currents 
of air; and if I now arrange the glass tube so as just to dip into this 
luminous part, as in our first experiment, only higher, you see the 

' Lycopodium is a yellowish powder found in the fruit of the club moss (Lycopo- 
dium clavatum). It is used in fireworks. 



CHEMICAL HISTORY OF A CANDLE IO7 

result. In place of having the same white vapor that you had before, 
you will now have a black vapor. There it goes, as black as ink. It 
is certainly very different from the white vapor; and when we put a 
light to it we shall find that it does not burn, but that it puts the 
light out. Well, these particles, as I said before, are just the smoke 
of the candle; and this brings to mind that old employment which 
Dean Swift recommended to servants for their amusement, namely, 
writing on the ceiling of a room with a candle. But what is that 
black substance.? Why, it is the same carbon which exists in the 
candle. How comes it out of the candle? It evidently existed in the 
candle, or else we should not have had it here. And now I want 
you to follow me in this explanation. You would hardly think that 
all those substances which fly about London, in the form of soots 
and blacks, are the very beauty and life of the flame, and wliich are 
burned in it as those iron filings were burned here. Here is a piece 
of wire gauze, which will not let the flame go through it; and I 
think you will see, almost immediately, that when I bring it low 
enough to touch that part of the flame which is otherwise so bright, 
it quells and quenches it at once, and allows a volume of smoke 
to rise up. 

I want you now to follow me in this point — that whenever a sub- 
stance burns, as the iron filings burnt in the flame of gunpowder, 
without assuming the vaporous state (whether it becomes liquid or 
remains solid), it becomes exceedingly luminous. I have here taken 
three or four examples apart from the candle on purpose to illus- 
trate this point to you, because what I have to say is applicable to 
all substances, whether they burn or whether they do not burn — that 
they are exceedingly bright if they retain their solid state, and that 
it is to this presence of soUd particles in the candle flame that it 
owes its brilliancy. 

Here is a platinum wire, a body which does not change by heat. 
If I heat it in this flame, see how exceedingly luminous it becomes. 
I will make the flame dim for the purpose of giving a litde light 
only, and yet you will see that the heat which it can give to that 
platinum wire, though far less than the heat it has itself, is able to 
raise the platinum wire to a far higher state of effulgence. This flame 



I08 FARADAY 

has carbon in it; but I will take one that has no carbon in it. There 
is a material, a kind of fuel — a vapor, or gas, whichever you like to 
call it — in that vessel, and it has no solid particles in it; so I take that 
because it is an example of flame itself burning without any solid 
matter whatever; and if I now put this solid substance in it, you see 
what an intense heat it has, and how brightly it causes the solid body 
to glow. This is the pipe through which we convey this particular 
gas, which we call hydrogen, and which you shall know all about 
the next time we meet. And here is a substance called oxygen, by 
means of which this hydrogen can burn; and although we produce, 
by their mixture, far greater heat (') than you can obtain from the 
candle, yet there is very little light. If, however, I take a solid 
substance, and put that into it, we produce an intense light. If I take 
a piece of lime, a substance which will not burn, and which will not 
vaporize by the heat (and because it does not vaporize remains solid, 
and remains heated), you will soon observe what happens as to its 
glowing. I have here a most intense heat produced by the burning 
of hydrogen in contact with the oxygen; but there is as yet very little 
light — not for want of heat, but for want of particles which can 
retain their solid state; but when I hold this piece of lime in the 
flame of the hydrogen as it burns in the oxygen, see how it glows! 
This is the glorious lime light, which rivals the voltaic light, and 
which is almost equal to sunlight. I have here a piece of carbon or 
charcoal, which will burn and give us light exactly in the same man- 
ner as if it were burnt as part of a candle. The heat that is in the 
flame of a candle decomposes the vapor of the wax, and sets free the 
carbon particles; they rise up heated and glowing as this now glows, 
and then enter into the air. But the particles, when burnt, never pass 
off from a candle in the form of carbon. They go off into the air as 
a perfectly invisible substance, about which we shall know hereafter. 
Is it not beautiful to think that such a process is going on, and 
that such a dirty thing as charcoal can become so incandescent ? You 
see it comes to this — that all bright flames contain these solid parti- 
cles; all things that burn and produce solid particles, either during 
the time they are burning, as in the candle, or immediately after 

' Bunsen has calculated that the temperature of the oxyhydrogen blowpipe is 8061° 
Centigrade. Hydrogen burning in air has a temperature of 3259° C, and coal gas 
in air, 2350° C. 




CHEMICAL HISTORY OF A CANDLE I Op 

being burnt, as in the case of the gunpowder and iron fiUngs — all 
these things give us this glorious and beautiful light. 

I will give you a few illustrations. Here is a piece of phosphorus, 
which burns with a bright flame. Very well; we may now conclude 
that phosphorus will produce, either at the moment that it is burn- 
ing or afterwards, these solid particles. Here is the phosphorus 
lighted, and I cover it over with this glass 
for the purpose of keeping in what is pro- 
duced. What is all that smoke ? (Fig. 63.) 
That smoke consists of those very particles 
which are produced by the combustion of 
the phosphorus. Here, again, are two sub- 
stances. This is chlorate of potassa, and this 
other sulphuret of antimony. I shall mix 
these together a little, and then they may 

be burnt in many ways. I shall touch them with a drop of sulphuric 
acid, for the purpose of giving you an illustration of chemical action, 
and they will instantly burn.(^) [The lecturer then ignited the mix- 
ture by means of sulphuric acid.] Now, from the appearance of 
things, you can judge for yourselves whether they produce solid 
matter in burning. I have given you the train of reasoning which 
will enable you to say whether they do or do not; for what is this 
bright flame but the solid particles passing off.'* 

Mr. Anderson has in the furnace a very hot crucible. I am about 
to throw into it some zinc filings, and they will burn with a flame 
like gunpowder. I make this experiment because you can make it 
well at home. Now I want you to see what will be the result of the 
combustion of this zinc. Here it is burning — burning beautifully like 
a candle, I may say. But what is all that smoke, and what are those 
little clouds of wool which will come to you if you can not come to 
them, and make themselves sensible to you in the form of the old 
philosophic wool, as it was called ? We shall have left in that crucible, 
also, a quantity of this woolly matter. But I will take a piece of this 

' The following is the action of the sulphuric in inflaming the mixture of sulphuret 
of antimony and chlorate of potassa. A portion of the latter is decomposed by the 
sulphuric acid into oxide of chlorine, bisulphate of potassa, and perchlorate of potassa. 
The oxide of chlorine inflames the sulphuret of antimony, which is a combustible 
body, and the whole mass instantly bursts into flame. 



no FARADAY 

same zinc, and make an experiment a little more closely at home, as 
it were. You will have here the same thing happening. Here is the 
piece of zinc; there [pointing to a jet of hydrogen] is the furnace, and 
we will set to work and try and burn the metal. It glows, you see; 
there is the combustion; and there is the white substance into which 
it burns. And so, if I take that flame of hydrogen as the representa- 
tive of a candle, and show you a substance like zinc burning in the 
flame, you will see that it was merely during the action of combus- 
tion that this substance glowed — while it was kept hot; and if I take 
a flame of hydrogen and put this white substance from the zinc into 
it, look how beautifully it glows, and just because it is a solid 
substance. 

I will now take such a flame as I had a moment since, and set free 
from it the particles of carbon. Here is some camphene, which will 
burn with a smoke; but if I send these particles of smoke through this 
pipe into the hydrogen flame you will see they will burn and become 
luminous, because we heat them a second time. There they are. 
Those are the particles of carbon reignited a second time. They are 
those particles which you can easily see by holding a piece of paper 
behind them, and which, while they are in the flame, are ignited by 
the heat produced, and, when so ignited, produce this brightness. 
When the particles are not separated you get no brightness. The 
flame of coal gas owes its brightness to the separation, during com- 
bustion, of these particles of carbon, which are equally in that as 
in a candle. I can very quickly alter that arrangement. Here, for 
instance, is a bright flame of gas. Supposing I add so much air to 
the flame as to cause it all to burn before those particles are set free, 
I shall not have this brightness; and I can do that in this way: If 
I place over the jet this wire-gauze cap, as you see, and then light 
the gas over it, it burns with a non-luminous flame, owing to its 
having plenty of air mixed with it before it burns; and if I raise the 
gauze, you see it does not burn below('°). There is plenty of carbon 

'"The "air-burner," which is of such value in the laboratory, owes its advantage 
to this principle. It consists of a cylindrical metal chimney, covered at the top with 
a piece of rather coarse iron wire gauze. This is supported over an Argand burner in 
such a manner that the gas may mix in the chimney with an amount of air sufficient 
to burn the carbon and hydrogen simultaneously, so that there may be no separation 
of carbon in the flame with consequent deposition of soot. The flame, being unable 
to pass through the wire gauze, burns in a steady, nearly invisible manner above. 



CHEMICAL HISTORY OF A CANDLE III 

in the gas; but, because the atmosphere can get to it, and mix with 
it before it burns, you see how pale and blue the flame is. And if I 
blow upon a bright gas flame, so as to consume all this carbon before 
it gets heated to the glowing point, it will also burn blue. [The lec- 
turer illustrated his remarks by blowing on the gas light.] The only 
reason why I have not the same bright light when I thus blow upon 
the flame is that the carbon meets with sufficient air to burn it be- 
fore it gets separated in the flame in a free state. The difference is 
solely due to the solid particles not being separated before the gas is 
burnt. 

You observe that there are certain products as the result of the com- 
bustion of a candle, and that of these products one portion may be 
considered as charcoal, or soot; that charcoal, when afterward burnt, 
produces some other product; and it concerns us very much now to 
ascertain what that other product is. We showed that something 
was going away; and I want you now to understand how much is 
going up into the air; and for that purpose we will have combustion 
on a litde larger scale. From that candle ascends heated air, and two 
or three experiments will show you the ascending current; but, in 
order to give you a notion of the quantity of matter which ascends in 
this way, I will make an experiment by which I shall try to imprison 
some of the products of this combustion. For this purpose I have here 
what boys call a fire-balloon; I use this fire-balloon merely as a sort 
of measure of the result of the combustion we are considering; and 
I am about to make a flame in such an easy and simple manner as 
shall best serve my present purpose. This plate shall be the "cup," 
we will so say, of the candle; this spirit shall be our fuel; and I am 
about to place this chimney over it, because it is better for me to do 
so than to let things proceed at random. Mr. Anderson will now 
light the fuel, and here at the top we shall get the results of the 
combustion. What we get at the top of that tube is exactly the 
same, generally speaking, as you get from the combustion of a 
candle; but we do not get a luminous flame here, because we use a 
substance which is feeble in carbon. I am about to put this balloon- 
not into action, because that is not my object — but to show you the 
effect which results from the action of those products which arise 
from the candle, as they arise here from the furnace. [The balloon 



112 



FARADAY 



was held over the chimney (Fig. 64), when it immediately com- 
menced to fill.] You see how it is disposed to ascend; but we must 
not let it up; because it might come in contact with those upper 
gaslights, and that would be very inconvenient. [The upper gas- 
lights were turned out at the request of the 
lecturer, and the balloon was allowed to 
ascend.] Does not that show you what a 
large bulk of matter is being evolved .'' Now 
there is going through this tube [placing a 
large glass tube over a candle] all the prod- 
ucts of that candle, and you will presently 
see that the tube will become quite opaque. 
Suppose I take another candle, and place it 
under a jar, and then put a light on the other 
side, just to show you what is going on. You 
see that the sides of the jar become cloudy, 
and the light begins to burn feebly. It is the 
products, you see, which make the light so 
dim, and this is the same thing which makes 
the sides of the jar so opaque. If you go 
home, and take a spoon that has been in the 
cold air, and hold it over a candle — not so as 
to soot it — you will find that it becomes dim 
just as that jar is dim. If you can get a silver dish, or something of 
that kind, you will make the experiment still better; and now, just 
to carry your thoughts forward to the time we shall next meet, let 
me tell you that it is water which causes the dimness, and when we 
next meet I will show you that we can make it, without difficulty, 
assume the form of a liquid. 




Fig. 64 



LECTURE III 

PRODUCTS: WATER FROM THE COMBUSTION- 
NATURE OF WATER— A COMPOUND- 
HYDROGEN 

I DARE say you well remember that when we parted we had 
just mentioned the word "products" from the candle; for when 
a candle burns we found we were able, by nice adjustment, to 
get various products from it. There was one substance which was 
not obtained when the candle was burning properly, which was 
charcoal or smoke, and there was some other substance that went 
upward from the flame which did not appear as smoke, but took 
some other form, and made part of that general current which, 
ascending from the candle upward, becomes invisible, and escapes. 
There were also other products to mention. You remember that 
in that rising current having its origin at the candle we found that 
one part was condensable against a cold spoon, or against a clean 
plate, or any other cold thing, and another part was incondensable. 

We will first take the condensable part, and examine it, and, 
strange to say, we find that that part of the product is just water — 
nothing but water. On the last occasion I spoke of it incidentally, 
merely saying that water was produced among the condensable 
products of the candle; but to-day I wish to draw your attention to 
water, that we may examine it carefully, especially in relation to this 
subject, and also with respect to its general existence on the surface 
of the globe. 

Now, having previously arranged an experiment for the purpose 
of condensing water from the products of the candle, my next point 
will be to show you this water; and perhaps one of the best means 
that I can adopt for showing its presence to so many at once is to 
exhibit a very visible action of water, and then to apply that test to 
what is collected as a drop at the bottom of that vessel. I have here 
a chemical substance, discovered by Sir Humphry Davy, which 

"3 



114 FARADAY 

has a very energetic action upon water, which I shall use as a test 
of the presence of water. If I take a little piece of it — it is called 
potassium, as coming from potash — if I take a little piece of it, and 
throw it into that basin, you see how it shows the presence of water 
by lighting up and floating about, burning with a violet flame. I 
am now going to take away the candle which has been burning 
beneath the vessel containing ice and salt, and you see a drop of 
water — a condensed product of the candle — hanging 
from the under surface of the dish (Fig. 65) . I will 
show you that potassium has the same action upon it 
as upon the water in that basin in the experiment we 
have just tried. See! it takes fire, and burns in just 
the same manner. I will take another drop upon this 
glass slab, and when I put the potassium on to it, you 
see at once, from its taking fire, that there is water 
present. Now that water was produced by the candle. 
In the same manner, if I put this spirit lamp under 
that jar, you will soon see the latter become damp 
from the dew which is deposited upon it — that dew 
being the result of combustion; and I have no doubt 
you will shortly see, by the drops of water which fall 
upon the paper below, that there is a good deal of 
water produced from the combustion of the lamp. I will let it remain, 
and you can afterward see how much water has been collected. So, if 
I take a gas lamp, and put any cooling arrangement over it, I shall 
get water — water being likewise produced from the combustion of 
gas. Here, in this bottle, is a quantity of water — perfectly pure, 
distilled water, produced from the combustion of a gas lamp — in no 
point different from the water that you distill from the river, or 
ocean, or spring, but exactly the same thing. Water is one individual 
thing; it never changes. We can add to it by careful adjustment for 
a little while, or we can take it apart and get other things from it; 
but water, as water, remains always the same, either in a solid, liquid, 
or fluid state. Here again [holding another bottle] is some water 
produced by the combustion of an oil lamp. A pint of oil, when 
burnt fairly and properly, produces rather more than a pint of 
water. Here, again, is some water, produced by a rather long experi- 




FiG. 65 



CHEMICAL HISTORY OF A CANDLE II5 

ment, from a wax candle. And so we can go on with almost all 
combustible substances, and find that if they burn with a flame, as 
a candle, they produce water. You may make these experiments 
yourselves: the head of a poker is a very good thing to try with, 
and if it remains cold long enough over the candle, you may get 
water condensed in drops on it; or a spoon, or ladle, or any thing 
else may be used, provided it be clean, and can carry ofl the heat, and 
so condense the water. 

And now — to go into the history of this wonderful production of 
water from combustibles, and by combustion — I must first of all 
tell you that this water may exist in different conditions; and 
although you may now be acquainted with all its forms, they still 
require us to give a little attention to them for the present; so that 
we may perceive how the water, while it goes through its Protean 
changes, is entirely and absolutely the same thing, whether it is 
produced from a candle, by combustion, or from the rivers or ocean. 

First of all, water, when at the coldest, is ice. Now we philosophers 
— I hope that I may class you and myself together in this case — 
speak of water as water, whether it be in its solid, or liquid, or 
gaseous state — we speak of it chemically as water. Water is a thing 
compounded of two substances, one of which we have derived from 
the candle, and the other we shall find elsewhere. Water may occur 
as ice; and you have had most excellent opportunities lately of seeing 
this. Ice changes back into water — for we had on our last Sabbath 
a strong instance of this change by the sad catastrophe which 
occurred in our own house, as well as in the houses of many of our 
friends — ice changes back into water when the temperature is raised; 
water also changes into steam when it is warmed enough. The 
water which we have here before us is in its densest state("); and, 
although it changes in weight, in condition, in form, and in many 
other qualities, it still is water; and whether we alter it into ice by 
cooling, or whether we change it into steam by heat, it increases in 
volume — in the one case very strangely and powerfully, and in the 
other case very largely and wonderfully. For instance, I will now 
take this tin cylinder, and pour a little water into it, and, seeing how 
much water I pour in, you may easily estimate for yourselves how 

" Water is in its densest state at a temperature of 39.1 ° Fahrenheit. 



Il6 FARADAY 

high it will rise in the vessel: it will cover the bottom about two 
inches. I am now about to convert the water into steam for the 
purpose of showing to you the different volumes which water 
occupies in its different states of water and steam. 

Let us now take the case of water changing into ice: we can effect 
that by cooling it in a mixture of salt and pounded ice('^) — and I 
shall do so to show you the expansion of water into a thing of 
larger bulk when it is so changed. These bottles [holding one] are 
made of strong cast iron, very strong and very thick — I suppose they 
are the third of an inch in thickness; they are very carefully filled 
with water, so as to exclude all air, and then they are screwed down 
tight. We shall see that when we freeze the water in these iron 
vessels, they will not be able to hold the ice, and the expansion within 
them will break them in pieces as these [pointing to some fragments] 
are broken, which have been bottles of exactly the same kind. I am 
about to put these two bottles into that mixture of ice and salt for 
the purpose of showing that when water becomes ice it changes in 
volume in this extraordinary way. 

In the mean time, look at the change which has taken place in 
the water to which we have applied heat; it is losing its fluid state. 
You may tell this by two or three circumstances. I have covered the 
mouth of this glass flask, in which water is boiling, with a watch- 
glass. Do you see what happens? It rattles away like a valve 
chattering, because the steam rising from the boiling water sends 
the valve up and down, and forces itself out, and so makes it clatter. 
You can very easily perceive that the flask is quite full of steam, or 
else it would not force its way out. You see, also, that the flask 
contains a substance very much larger than the water, for it fills 
the whole of the flask over and over again, and there it is blowing 
away into the air; and yet you can not observe any great diminution 
in the bulk of the water, which shows you that its change of bulk 
is very great when it becomes steam. 

I have put our iron bottles containing water into this freezing mix- 
ture, that you may see what happens. No communication will take 
place, you observe, between the water in the bottles and the ice in 

'^A mixture of salt and pounded ice reduces the temperature from 32° F. to zero, 
the ice at the same time becoming fluid. 



CHEMICAL HISTORY OF A CANDLE II 7 

the outer vessel. But there will be a conveyance of heat from the 
one to the other, and if we are successful — we are making our 
experiment in very great haste — I expect you will by-and-by, so soon 
as the cold has taken possession of the bottles and their contents, 
hear a pop on the occasion of the bursting of the one bottle or the 
other, and, when we come to examine the bottles, we shall find their 
contents masses of ice, partly inclosed by the covering of iron which 
is too small for them, because the ice is larger in bulk than the 





Fig. 66 

water. You know very well that ice floats upon water; if a boy falls 
through a hole into the water, he tries to get on the ice again to 
float him up. Why does the ice float? Think of that, and philoso- 
phize. Because the ice is larger than the quantity of water which 
can produce it, and therefore the ice weighs the lighter and the 
water is the heavier. 

To return now to the action of heat on water. See what a stream 
of vapor is issuing from this tin vessel! You observe, we must have 
made it quite full of steam to have it sent out in that great quantity. 
And now, as we can convert the water into steam by heat, we con- 
vert it back into liquid water by the application of cold. And if we 



Il8 FARADAY 

take a glass, or any other cold thing, and hold it over this steam, 
see how soon it gets damp with water: it will condense it until the 
glass is warm — it condenses the water which is now running down 
the sides of it. I have here another experiment to show the con- 
densation of water from a vaporous state back into a liquid state, 
in the same way as the vapor, one of the products of the candle, was 
condensed against the bottom of the dish and obtained in the form 
of water; and to show you how truly and thoroughly these changes 
take place, I will take this tin flask, which is now full of steam, 
and close the top. We shall see what takes place when we cause this 
water or steam to return back to the fluid state by pouring some cold 
water on the outside. [The lecturer poured the cold water over the 
vessel, when it immediately collapsed (Fig. 66).] You see what has 
happened. If I had closed the stopper, and still kept the heat applied 
to it, it would have burst the vessel; yet, when the steam returns to 
the state of water, the vessel collapses, there being a vacuum pro- 
duced inside by the condensation of the steam. I show you these 
experiments for the purpose of pointing out that in all these occur- 
rences there is nothing that changes the water into any other thing; 
it still remains water; and so the vessel is obliged to give way, and 
is crushed inward, as in the other case, by the farther application of 
heat, it would have been blown outward. 

And what do you think the bulk of that water is when it assumes 
the vaporous condition.'' You see that cube [pointing to a cubic 

foot]. There, by its side, is a cubic 
inch (Fig. 67), exactly the same shape 
as the cubic foot, and that bulk of 
water [the cubic inch] is sufficient to 
expand into that bulk [the cubic foot] 
of steam; and, on the contrary, the 
application of cold will contract that 
large quantity of steam into this 
'°' ^ small quantity of water. [One of the 

iron bottles burst at that moment.] Ah! There is one of our bottles 
burst, and here, you see, is a crack down one side an eighth of an 
inch in width. [The other now exploded, sending the freezing 
mixture in all directions.] This other Ijottle is also broken; although 




CHEMICAL HISTORY OF A CANDLE II 9 

the iron was nearly half an inch thick, the ice has burst it asunder. 
These changes always take place in water; they do not require to be 
always produced by artificial means; we only use them here because 
we want to produce a small winter round that little bottle instead 
of a long and severe one. But if you go to Canada, or to the North, 
you will find the temperature there out of doors will do the same 
thing as has been done here by the freezing mixture. 

To return to our quiet philosophy. We shall not in future be 
deceived, therefore, by any changes that are produced in water. 
Water is the same every where, whether produced from the ocean or 
from the flame of the candle. Where, then, is this water which we 
get from a candle? I must anticipate a little, and tell you. It 
evidently comes, as to part of it, from the candle, but is it within the 
candle beforehand? No, it is not in the candle; and it is not in the 
air around about the candle which is necessary for its combustion. 
It is neither in one nor the other, but it comes from their conjoint 
action, a part from the candle, a part from the air; and this we 
have now to trace, so that we may understand thoroughly what is 
the chemical history of a candle when we have it burning on our 
table. How shall we get at this? I myself know plenty of ways, 
but I want you to get at it from the association in your own minds 
of what I have already told you. 

I think you can see a little in this way. We had just now the 
case of a substance which acted upon the water in the way that 
Sir Humphry Davy showed us('^), and which I am now going to 
recall to your minds by making again an experiment upon that dish. 
It is a thing which we have to handle very carefully; for you see, if 
I allow a little splash of water to come upon this mass, it sets fire 
to part of it; and if there were free access of air, it would quickly set 
fire to the whole. Now this is a metal — a beautiful and bright 
metal — which rapidly changes in the air, and, as you know, rapidly 
changes in water. I will put a piece on the water, and you see it 
burns beautifully, making a floating lamp, using the water in the 
place of air. Again, if we take a few iron filings or turnings and 

"Potassium, the metallic basis of potash, was discovered by Sir Humphry Davy 
in 1807, who succeeded in separating it from potash by means of a powerful voltaic 
battery. Its great affinity for oxygen causes it to decompose water with evolution of 
hydrogen, which takes fire with the heat produced. 



120 FARADAY 

put them in water, we find that they Ukewise undergo an alteration. 
They do not change so much as this potassium does, but they change 
somewhat in the same way; they become rusty, and show an action 
upon the water, though in a different degree of intensity to what 
this beautiful metal does; but they act upon the water in the same 
manner generally as this potassium. I want you to put these different 
facts together in your minds. I have another metal here [zinc], 
and when we examined it with regard to the solid substance pro- 
duced by its combustion, we had an opportunity of seeing that it 
burned; and I suppose, if I take a little strip of this zinc and put it 
over the candle, you will see something half way, as it were, between 
the combustion of potassium on the water and the action of iron — 
you see there is a sort of combustion. It has burned, leaving a white 
ash or residuum; and here also we find that the metal has a certain 
amount of action upon water. 

By degrees we have learned how to modify the action of these 
different substances, and to make them tell us what we want to 
know. And now, first of all, I take iron. It is a common thing in 
all chemical reactions, where we get any result of this kind, to find 
that it is increased by the action of heat; and if we want to examine 
minutely and carefully the action of bodies one upon another, we 
often have to refer to the action of heat. You are aware, I believe, 
that iron filings burn beautifully in the air; but I am about to show 
you an experiment of this kind, because it will impress upon you 
what I am going to say about iron in its action on water. If I take a 
flame and make it hollow — you know why, because I want to get 
air to it and into it, and therefore I make it hollow — and then take 
a few iron filings and drop them into the flame, you see how well 
they burn. That combustion results from the chemical action which 
is going on when we ignite those particles. And so we proceed to 
consider these different effects, and ascertain what iron will do when 
it meets with water. It will tell us the story so beautifully, so gradu- 
ally and regularly, that I think it will please you very much. 

I have here a furnace with a pipe going through it like an iron 
gun barrel (Fig. 68), and I have stuffed that barrel full of bright 
iron turnings, and placed it across the fire to be made red-hot. We 
can either send air through the barrel to come in contact with the 



CHEMICAL HISTORY OF A CANDLE 



121 



iron, or we can send steam from this little boiler at the end of the 
barrel. Here is a stop-cock which shuts off the steam from the 
barrel until we wish to admit it. There is some water in these glass 
jars, which I have colored blue, so that you may see what happens. 
Now you know very well that any steam I might send through that 
barrel, if it went through into the water, would be condensed; for 
you have seen that steam can not retain its gaseous form if it be 
cooled down; you saw it here [pointing to the tin flask] crushing 
itself into a small bulk, and causing the flask holding it to collapse; 




Fig. 68 

SO that if I were to send steam through that barrel it would be con- 
densed, supposing the barrel were cold; it is, therefore, heated to 
perform the experiment I am now about to show you. I am going 
to send the steam through the barrel in small quantities, and you 
shall judge for yourselves, when you see it issue from the other end, 
whether it still remains steam. Steam is condensible into water, and 
when you lower the temperature of steam you convert it back into 
fluid water; but I have lowered the temperature of the gas which 
I have collected in this jar by passing it through water after it has 
traversed the iron barrel, and still it does not change back into water. 
I will take another test and apply to this gas. (I hold the jar in an 
inverted position, or my substance would escape.) If I now apply 
a light to the mouth of the jar, it ignites with a slight noise. That 
tells you that it is not steam; steam puts out a fire: it does not burn; 
but you saw that what I had in that jar burnt. We may obtain this 



122 



FARADAY 



substance equally from water produced from the candle flame as 
from any other source. When it is obtained by the action of the 
iron upon the aqueous vapor, it leaves the iron in a state very 
similar to that in which these filings were after they were burnt. It 
makes the iron heavier than it was before. So long as the iron 
remains in the tube and is heated, and is cooled again without the 
access of air or water, it does not change in its weight; but after 
having had this current of steam passed over it, it then comes out 
heavier than it was before, having taken something out of the 
steam, and having allowed something else to pass forth, which we 
see here. And now, as we have another jar full, I will show you 
something most interesting. It is a combustible gas; and I might at 
once take this jar and set fire to the contents, and show you that it 
is combustible; but I intend to show you more, if I can. It is also 
a very light substance. Steam will condense; this body will rise in 
the air, and not condense. Suppose I take another glass jar, empty 
of all but air: if I examine it with a taper I shall find that it con- 
tains nothing but air. I will now take this jar full of the gas that 
I am speaking of, and deal with it as though it were a light body; 

I will hold both upside down, and turn 
the one up under the other (Fig. 69); 
and that which did contain the gas pro- 
cured from the steam, what does it con- 
tain now? You will find it now only 
contains air. But look! Here is the com- 
bustible substance [taking the other jar] 
which I have poured out of the one jar 
^'°- ^9 into the other. It still preserves its qual- 

ity, and condition, and independence, and therefore is the more 
worthy of our consideration, as belonging to the products of a candle. 
Now this substance which we have just prepared by the action 
of iron on the steam or water, we can also get by means of those 
other things which you have already seen act so well upon the 
water. If I take a piece of potassium, and make the necessary 
arrangements, it will produce this gas; and if, instead, a piece of 
zinc, I find, when I come to examine it very carefully, that the main 
reason why this zinc can not act upon the water continuously as 




CHEMICAL HISTORY OF A CANDLE 1 23 

the other metal does, is because the result of the action of the water 
envelops the zinc in a kind of protecting coat. We have learned in 
consequence, that if we put into our vessel only the zinc and water, 
they, by themselves, do not give rise to much action, and we get no 
result. But suppose I proceed to dissolve off this varnish — this 
encumbering substance — which I can do by a little acid; the moment 
I do this I find the zinc acting upon the water exactly as the iron 
did, but at the common temperature. The acid in no way is altered, 
except in its combination with the oxide of zinc which is produced. 
I have now poured the acid into the glass, and the effect is as though 
I were applying heat to cause this boiling up. There is something 
coming off from the zinc very abundantly, which is not steam. 
There is a jar full of it; and you will find that I have exactly the 
same combustible substance remaining in the vessel, when I hold it 
upside down, that I produced during the experiment with the iron 
barrel. This is what we get from water, the same substance which 
is contained in the candle. 

Let us now trace distinctly the connection between these two 
points. This is hydrogen — a body classed arnong those things which 







' 




llj^f^ 






l\ ffe^ 




^ 


^^^^^te:' 


W 


^\ 




^\ 




-^r^ ""'*\=^\^^- 


— ^ 


:rr\ 


\. n^^ 


\ 


rr 


- ■ — ■ — 'fc .- .' \ — 't- 


w 


■ — N;^ 


\ 


U- - — ^^ ■§ 


1 


■ . \ 




Fig. 70 

in chemistry we call elements, because we can get nothing else out 
of them. A candle is not an elementary body, because we can get 
carbon out of it; we can get this hydrogen out of it, or at least out of 
the water which it supplies. And this gas has been so named 
hydrogen, because it is that element which, in association with 
another, generates water.* Mr. Anderson having now been able to 
get two or three jars of gas, we shall have a few experiments to 
make, and I want to show you the best way of making these experi- 

*''TSup, "water," and yemau, "1 generate." 



124 FARADAY 

ments. I am not afraid to show you, for I wish you to make ex- 
periments, if you will only make them with care and attention, and 
the assent of those around you. As we advance in chemistry we are 
obliged to deal with substances which are rather injurious if in 
their wrong places; the acids, and heat, and combustible things we 
use, might do harm if carelessly employed. If you want to make 
hydrogen, you can make it easily from bits of zinc, and sulphuric 
or muriatic acid. Here is what in former times was called the 
"philosopher's candle." It is a little phial with a cork and a tube or 
pipe passing through it. And I am now putting a few little pieces 
of zinc into it. This little instrument I am going to apply to a useful 
purpose in our demonstrations, for I want to show you that you can 
prepare hydrogen, and make some experiments with it as you please, 
at your own homes. Let me here tell you why I am so careful to 
fill this phial nearly, and yet not quite full. I do it because the 
evolved gas, which, as you have seen, is very combustible, is explosive 
to a considerable extent when mixed with air, and might lead to 
harm if you were to apply a light to the end of that pipe before all 
the air had been swept out of the space above the water. I am now 
about to pour in the sulphuric acid. I have used very little zinc and 
more sulphuric acid and water, because I want to keep it at work 
for some time. I therefore take care in this way to modify 
the proportions of the ingredients so that I may have a 
regular supply — not too quick and not too slow. Supposing 
I now take a glass and put it upside down over the end of 
the tube, because the hydrogen is light I expect that it will 
remain in that vessel a little while. We will now test the 
contents of our glass to see if there be hydrogen in it; I 
think I am safe in saying we have caught some [applying 
a light]. There it is, you see. I will now apply a light to the 
top of the tube. There is the hydrogen burning (Fig. 71). 
There is our philosophical candle. It is a foolish, feeble sort 
of a flame, you may say, but it is so hot that scarcely any 
common flame gives out so much heat. It goes on burning 
regularly, and I am now about to put that flame to burn under a 
certain arrangement, in order that we may examine its results and 
make use of the information which we may thereby acquire. Inas- 



CHEMICAL HISTORY OF A CANDLE 1 25 

much as the candle produces water, and this gas comes out o£ the 
water, let us see what this gives us by the same process of combustion 
that the candle went through when it burnt in the atmosphere; and 
for that purpose I am going to put the lamp under this apparatus 
(Fig. 72), in order to condense whatever may arise from the com- 

^ bustion within it. In 

J,ff/'.('''^\^r "■- '■ ^^"""-^—^ the course of a short 

time you will see mois- 
ture appearing in the 
cylinder, and you will get the water running down the 
side, and the water from this hydrogen flame will have 
absolutely the same effect upon all our tests, being obtained 
by the same general process as in the former case. This 
hydrogen is a very beautiful substance. It is so light that 
it carries things up; it is far lighter than the atmosphere; 
Fio. 72 gjjj J j^^g 5^y J (,^jj show you this by an experiment which, 
if you are very clever, some of you may even have skill enough to re- 
peat. Here is our generator of hydrogen, and here are some soapsuds. 
I have an India-rubber tube connected with the hydrogen generator, 
and at the end of the tube is a tobacco pipe. I can thus put the pipe 
into the suds and blow bubbles by means of the hydrogen. You ob- 
serve how the bubbles fall downward when I blow them with my 
warm breath; but notice the difference when I blow them with hy- 
drogen. [The lecturer here blew bubbles with hydrogen, which rose 
to the roof of the theatre.] It shows you how light this gas must be in 
order to carry with it not merely the ordinary soap bubble, but the 
larger portion of a drop hanging to the bottom of it. I can show its 
lightness in a better way than this; larger bubbles than these may be 
so lifted up; indeed, in former times balloons used to be filled with 
this gas. Mr. Anderson will fasten this tube on to our generator, 
and we shall have a stream of hydrogen here with which we can 
charge this balloon made of collodion. I need not even be very 
careful to get all the air out, for I know the power of this gas to 
carry it up. [Two collodion balloons were inflated and sent up, one 
being held by a string.] Here is another larger one, made of thin 
membrane, which we will fill and allow to ascend; you will see 
they will all remain floating about until the gas escapes. 



126 FARADAY 

What, then, are the comparative weights of these substances? I 
have a table here which will show you the proportion which their 
weights bear to each other. I have taken a pint and a cubic foot as 
the measures, and have placed opposite to them the respective figures. 
A pint measure of this hydrogen weighs three-quarters of our 
smallest weight, a grain, and a cubic foot weighs one-twelfth of an 
ounce; whereas a pint of water weighs 8,750 grains, and a cubic foot 
of water weighs almost 1,000 ounces. You see, therefore, what a vast 
difference there is between the weight of a cubic foot of water and 
a cubic foot of hydrogen. 

Hydrogen gives rise to no substance that can become solid, either 
during combustion or afterward as a product of its combustion; 
but when it burns it produces water only; and if we take a cold glass 
and put it over the flame, it becomes damp, and you have water 
produced immediately in appreciable quantity; and nothing is pro- 
duced by its combustion but the same water which you have seen 
the flame of the candle produce. It is important to remember that 
this hydrogen is the only thing in nature which furnishes water as 
the sole product of combustion. 

And now we must endeavor to find some additional proof of the 
general character and composition of water, and for this purpose I 
will keep you a little longer, so that at our next meeting we may 
be better prepared for the subject. We have the power of arranging 
the zinc which you have seen acting upon the water by the assist- 
ance of an acid, in such a manner as to cause all the power to be 
evolved in the place where we require it. I have behind me a 
voltaic pile, and I am just about to show you, at the end of this 
lecture, its character and power, that you may see what we shall have 
to deal with when next we meet. I hold here the extremities of the 
wires which transport the power from behind me, and which I 
shall cause to act on the water. 

We have previously seen what a power of combustion is possessed 
by the potassium, or the zinc, or the iron filings; but none of them 
show such energy as this. [The lecturer here made contact between 
the two terminal wires of the battery, when a brilliant flash of light 
was produced.] This light is, in fact, produced by a forty-zinc 
power of burning; it is a power that I can carry about in my hands 



CHEMICAL HISTORY OF A CANDLE IT] 

through these wires at pleasure, although if I applied it wrongly to 
myself, it would destroy me in an instant, for it is a most intense 
thing, and the power you see here put forth while you count five 
[bringing the poles in contact and exhibiting the electric light] is 
equivalent to the power of several thunder-storms, so great is its 
force("). And that you may see what intense energy it has, I will 
take the ends of the wires which convey the power from the battery, 
and with it I dare say I can burn this iron file. Now this is a 
chemical power, and one which, when we next meet, I shall apply to 
water, and show you what results we are able to produce. 

'* Professor Faraday has calculated that there is as much electricity required to de- 
compose one grain of water as there is in a very powerful flash of lightning. 



LECTURE IV 

HYDROGEN IN THE CANDLE— BURNS INTO 

WATER— THE OTHER PART OF 

WATER— OXYGEN 

I SEE you are not tired of the candle yet, or I am sure you 
would not be interested in the subject in the way you are. When 
our candle was burning we found it produced water exactly 
like the water we have around us; and by farther examination of 
this water we found in it that curious body, hydrogen — that light 
substance of which there is some in this jar. We afterward saw the 
burning powers of that hydrogen, and that it produced water. And 
I think I introduced to your notice an apparatus which I very 
briefly said was an arrangement of chemical force, or power, or 
energy, so adjusted as to convey its power to us in these wires; and 
I said I should use that force to pull the water to pieces, to see what 
else there was in the water besides hydrogen; because, you remember, 
when we passed the water through the iron tube, we by no means 
got the weight of water back which we put in in the form of steam, 
though we had a very large quantity of gas evolved. We have now 
to see what is the other substance present. That you may under- 
stand the character and use of this instrument, let us make an 
experiment or two. Let us put together, first of all, some sub- 
stances, knowing what they are, and then see what that instrument 
does to them. There is some copper (observe the various changes 
which it can undergo), and here is some nitric acid, and you will 
find that this, being a strong chemical agent, will act very power- 
fully when I add it to the copper. It is now sending forth a beautiful 
red vapor; but as we do not want that vapor, Mr. Anderson will 
hold it near the chimney for a short time, that we may have the use 
and beauty of the experiment without the annoyance. The copper 
which I have put into the flask will dissolve it: it will change the 
acid and the water into a blue fluid containing copper and other 

128 



CHEMICAL HISTORY OF A CANDLE 1 29 

things, and I purpose then showing you how this voltaic battery 
deals with it; and in the mean time we will arrange another kind of 
experiment for you to see what power it has. This is a substance 
which is to us like water — that is to say, it contains bodies which 
we do not know of as yet, as water contains a body which we do 
not know as yet. Now this solution of a salt('^) I will put upon 
paper, and spread about, and apply the power of the battery to it, and 
observe what will happen. Three or four important things will 
happen which we shall take advantage of. I place this wetted paper 
upon a sheet of tin foil, which is convenient for keeping all clean, 
and also for the advantageous application of the power; and this 
solution, you see, is not at all affected by being put upon the paper 
or tin foil, nor by any thing else I have brought in contact with it 
yet, and which, therefore, is free to us to use as regards that instru- 
ment. But first let us see that our instrument is in order. Here are 
our wires. Let us see whether it is in the state in which it was last 
time. We can soon tell. As yet, when I bring them together, we 
have no power, because the conveyers — what we call the electrodes — 
the passages or ways for the electricity — are stopped; but now Mr. 
Anderson by that [referring to a sudden flash at the ends of the 
wires] has given me a telegram to say that it is ready. Before I begin 
our experiment I will get Mr. Anderson to break contact again at 
the battery behind me, and we will put a platinum wire across to 
connect the poles, and then if I find I can ignite a pretty good length 
of this wire we shall be safe in our experiment. Now you will see 
the power. [The connection was established, and the intermediate 
wire became red-hot.] There is the power running beautifully 
through the wire, which I have made thin on purpose to show you 
that we have those powerful forces; and now, having that power, we 
will proceed with it to the examination of water. 

I have here two pieces of platinum, and if I lay them down upon 
this piece of paper [the moistened paper on the tin foil] you will see 
no action; and if I take them up there is no change that you can 
see, but the arrangement remains just as it was before. But, now, 

'^ A solution of acetate of lead submitted to the action of the voltaic current yields 
lead at the negative pole, and brown peroxide of lead at the positive pole. A solution 
of nitrate of silver, under the same circumstances, yields silver at the negative pole, 
and peroxide of silver at the positive pole. 



130 FARADAY 

see what happens: if I take these two poles and put either one or 
the other of them down separately on the platinum plates, they do 
nothing for me; both are perfectly without action; but if I let them 
both be in contact at the same moment, see what happens [a brown 
spot appeared under each pole of the battery]. Look here at the 
effect that takes place, and see how I have pulled something apart 
from the white — something brown; and I have no doubt, if I were 
to arrange it thus, and were to put one of the poles to the tin foil 
on the other side of the paper — why, I get such a beautiful action 
upon the paper that I am going to see whether I can not write with 
it — a telegram, if you please. [The lecturer here traced the word 
"juvenile" on the paper with one of the terminal wires.] See there 
how beautifully we can get our results! 

You see we have here drawn something which we have not 
known about before out of this solution. Let us now take that 
flask from Mr. Anderson's hands, and see what we can draw out of 
that. This, you know, is a liquid which we have just made up from 
copper and nitric acid while our other experiments were in hand; 
and though I am making this experiment very hastily, and may 
bungle a little, yet I prefer to let you see what I do rather than pre- 
pare it beforehand. 

Now see what happens. These two platinum plates are the two 
ends (or I will make them so immediately) of this apparatus; and 
I am about to put them in contact with that solution, just as we 
did a moment ago on the paper. It does not matter to us whether the 
solution be on the paper or whether it be in the jar, so long as we 
bring the ends of the apparatus to it. If I put the two platinums 
in by themselves they come out as clean and as white as they go in 
[inserting them into the fluid without connecting them with the 
battery] ; but when we take the power and lay that on [the platinums 
were connected with the battery and again dipped into the solution], 
this, you see [exhibiting one of the platinums], is at once turned into 
copper, as it were; it has become like a plate of copper; and that 
[exhibiting the other piece of platinum] has come out quite clean. 
If I take this coppered piece and change sides, the copper will leave 
the right-hand side and come over to the left side; what was before 
the coppered plate comes out clean, and the plate which was clean 



CHEMICAL HISTORY OF A CANDLE I3I 

comes out coated with copper; and thus you see that the same copper 
we put into this solution we can also take out of it by means of this 
instrument. 

Putting that solution aside, let us now see what effect this instru- 
ment will have upon water (Fig. 73). Here are two little platinum 
plates which I intend to make the ends of the battery, and this (C) 




Fig. 73 

is a little vessel so shaped as to enable me to take it to pieces, and 
show you its construction. In these two cups (A and B) I pour 
mercury, which touches the ends of the wires connected with the 
platinum plates. In the vessel (C) I pour some water containing a 
little acid (but which is put in only for the purpose of facilitating 
the action; it undergoes no change in the process), and connected 
with the top of the vessel is a bent glass tube (D), which may 
remind you of the pipe which was connected with the gun barrel 
in our furnace experiment, and which now passes under the jar 
(F). I have now adjusted this apparatus, and we will proceed to 
affect the water in some way or other. In the other case I sent the 
water through a tube which was made red-hot; I am now going to 
pass the electricity through the contents of this vessel. Perhaps I may 
boil the water; if I do boil the water, I shall get steam; and you 
know that steam condenses when it gets cold, and you will there- 
fore see by that whether I do boil the water or not. Perhaps, how- 
ever, I shall not boil the water, but produce some other effect. You 
shall have the experiment and see. There is one wire which I will 



132 FARADAY 

put to this side (A), and here is the other wire which I will put to 
the other side (B), and you will soon see whether any disturbance 
takes place. Here it is seeming to boil up famously; but does it boil? 
Let us see whether that which goes out is steam or not. I think you 
will soon see the jar (F) will be filled with vapor, if that which rises 
from the water is steam. But can it be steam? Why, certainly not; 
because there it remains, you see, unchanged. There it is standing 
over the water, and it can not therefore be steam, but must be a 
permanent gas of some sort. What is it? Is it hydrogen? Is it 
anything else? Well, we will examine it. If it is hydrogen it will 
burn. [The lecturer then ignited a portion of the gas collected, 
which burnt with an explosion.] It is certainly something combus- 
tible, but not combustible in the way that hydrogen is. Hydrogen 
would not have given you that noise; but the color of that light, 
when the thing did burn, was like that of hydrogen; it will, however, 
burn without contact with the air. That is why I have chosen this 
other form of apparatus, for the purpose of pointing out to you 
what are the particular circumstances of this experiment. In place 
of an open vessel, I have taken one that is closed (our battery is so 
beautifully active that we are even boiling the mercury, and getting 
all things right — not wrong, but vigorously right) ; and I am going 
to show you that that gas, whatever it may be, can burn without 
air, and in that respect differs from a candle, which can not burn 
without the air. And our manner of doing this is as follows : I have 
here a glass vessel (G) which is fitted with two platinum wires 
(I K) through which I can apply electricity; and we can put the 
vessel on the air pump and exhaust the air; and when we have taken 
the air out we can bring it here and fasten it on to this jar (F), 
and let into the vessel that gas which was formed by the action of 
the voltaic battery upon the water, and which we have produced by 
changing the water into it; for I may go as far as this, and say we 
have really, by that experiment, changed the water into that gas. 
We have not only altered its condition, but we have changed it 
really and truly into that gaseous substance, and all the water is there 
which was decomposed by the experiment. As I screw this vessel 
(G H) on here (H), and make the tubes well connected, and when 
I open the stop-cocks (H H H), if you watch the level of the water 



CHEMICAL HISTORY OF A CANDLE I33 

(in F), you will see that the gas will rise. I will now close the stop- 
cocks, as I have drawn up as much as the vessel can hold, and being 
safely conveyed into that chamber, I will pass into it an electric spark 
from this Leyden jar (L), when the vessel, which is now quite clear 
and bright, will become dim. There will be no sound, for the 
vessel is strong enough to confine the explosion. [A spark was then 
passed through the jar, when the explosive mixture was ignited.] 
Did you see that brilliant light? If I again screw the vessel on to 
the jar, and open these stop-cocks, you will see that the gas will rise 
a second time. [The stop-cocks were then opened.] Those gases 
[referring to the gases first collected in the jar, and which had just 
been ignited by the electric spark] have disappeared, as you see; 
their place is vacant, and fresh gas has gone in. Water has been 
formed from them; and if we repeat our operation [repeating the 
last experiment], I shall have another vacancy, as you will see by the 
water rising. I always have an empty vessel after the explosion, 
because the vapor or gas into which that water has been resolved by 
the battery explodes under the influence of the spark, and changes 
into water; and by-and-by you will see in this upper vessel some 
drops of water trickling down the sides and collecting at the bottom. 

We are here dealing with water entirely, without reference to the 
atmosphere. The water of the candle had the atmosphere helping to 
produce it; but in this way it can be produced independently of the 
air. Water, therefore, ought to contain that other substance which 
the candle takes from the air, and which, combining with the hydro- 
gen, produces water. 

Just now you saw that one end of this battery took hold of the 
copper, extracting it from the vessel which contained the blue solu- 
tion. It was effected by this wire; and surely we may say, if the 
battery has such power with a metallic solution which we made and 
unmade, may we not find that it is possible to split asunder the com- 
ponent parts of the water, and put them into this place and that 
place? Suppose I take the poles — the metallic ends of this battery — 
and see what will happen with the water in this apparatus (Fig. 74), 
where we have separated the two ends far apart. I place one here 
(at A), and the other there (at B); and I have Httle shelves with 
holes which I can put upon each pole, and so arrange them that 




Fig. 74 



134 FARADAY 

whatever escapes from the two ends of the battery will appear as 
separate gases; for you saw that the water did not become vaporous, 
but gaseous. The wires are now in perfect and proper connection 
with the vessel containing the water, and you see the bubbles rising; 
let us collect these bubbles and see what they are. Here is a glass 

cylinder (O); I fill it with water and 
put it over one end (A) of the pile, and 
I will take another (H), and put it 
over the other end (B) of the pile. And 
so now we have a double apparatus, 
with both places delivering gas. Both 
these jars will fill with gas. There they 
go, that to the right (H) filling very 
rapidly; the one to the left (O) filling 
not so rapidly; and, though I have al- 
lowed some bubbles to escape, yet still 
the action is going on pretty regularly; 
and were it not that one is rather smaller than the other, you would 
see that I should have twice as much in this (H) as I have in that 
(O). Both these gases are colorless; they stand over the water with- 
out condensing; they are alike in all things — I mean in all apparent 
things; and we have here an opportunity of examining these bodies 
and ascertaining what they are. Their bulk is large, and we can 
easily apply experiments to them. I will take this jar (H) first, and 
will ask you to be prepared to recognize hydrogen. 

Think of all its qualities — the light gas which stood well in in- 
verted vessels, burning with a pale flame at the mouth of the jar, 
and see whether this gas does not satisfy all these conditions. If it be 
hydrogen it will remain here while I hold this jar inverted. [A light 
was then applied, when the hydrogen burnt.] What is there now in 
the other jar? You know that the two together made an explosive 
mixture. But what can this be which we find as the other constituent 
in water, and which must therefore be that substance which made 
the hydrogen burn ? We know that the water we put into the vessel 
consisted of the two things together. We find one of these is hydro- 
gen: what must that other be which was in the water before the 
experiment, and which we now have by itself.? I am about to put 



CHEMICAL HISTORY OF A CANDLE I35 

this lighted splinter of wood into the gas. The gas itself will not 
burn, but it will make the splinter of wood burn. [The lecturer 
ignited the end of the wood and introduced it into the jar of gas.] 
See how it invigorates the combustion of the wood, and how it 
makes it burn far better than the air would make it burn; and now 
you see by itself that every other substance which is contained in the 
water, and which, when the water was formed by the burning of the 
candle, must have been taken from the atmosphere. What shall we 
call it, A, B, or C? Let us call it O — call it "Oxygen"; it is a very 
good, distinct-sounding name. This, then, is the oxygen which was 
present in the water, forming so large a part of it. 

We shall now begin to understand more clearly our experiments 
and researches, because when we have examined these things once 
or twice we shall soon see why a candle burns in the air. When we 
have in this way analyzed the water — that is to say, separated or 
electrolyzed its parts out of it, we get two volumes of hydrogen and 
one of the body that burns it. And these two are represented to us 
on the following diagram, with their weights also stated; and we 
shall find that the oxygen is a very heavy body by comparison with 
the hydrogen. It is the other element in water. 



I 
Hydrogen 


8 
Oxygen 


9 



Oxygen . 
Hydrogen 

Water 



88.9 
II. I 



I had better, perhaps, tell you now how we get this oxygen abund- 
antly, having shown you how we can separate it from the water. 
Oxygen, as you will immediately imagine, exists in the atmosphere; 
for how should the candle burn to produce water without it ? Such 
a thing would be absolutely impossible, and chemically impossible, 
without oxygen. Can we get it from the air ? Well, there are some 
very complicated and difficult processes by which we can get it from 
the air; but we have better processes. There is a substance called 



136 FARADAY 

the black oxide of manganese; it is a very black-looking mineral, but 
very useful, and when made red-hot it gives out oxygen. Here is an 
iron bottle which has had some of this substance put into it, and 
there is a tube fixed to it (Fig. 75), and a iire ready made, and Mr. 
Anderson will put that retort into the fire, for it is made of iron, and 

can stand the heat. Here is a 
salt called chlorate of potassa, 
which is now made in large quantities for bleach- 
ing, and chemical and medical uses, and for pyro- 
technic and other purposes. I will take some and 
mix it with some of the oxide of manganese (oxide 
of copper, or oxide of iron would do as well) ; and 
if I put these together in a retort, far less than a red 
heat is sufficient to evolve this oxygen from the mix- 
ture. I am not preparing to make much, because we 
only want sufBcient for our experiments; only, as 
■'''°' ^5 you will see immediately, if I use too small a charge, 

the first portion of the gas will be mixed with the air already in the 
retort, and I should be obliged to sacrifice the first portion of the gas 
because it would be so much diluted with air; the first portion must 
therefore be thrown away. You will find in this case that a common 
spirit lamp is quite sufficient for me to get the oxygen, and so we shall 
have two processes going on for its preparation. See how freely the 
gas is coming over from that small portion of the mixture. We will 
examine it and see what are its properties. Now in this way we are 
producing, as you will observe, a gas just like the one we had in 
the experiment with the battery, transparent, undissolved by water, 
and presenting the ordinary visible properties of the atmosphere. 
(As this first jar contains the air, together with the first portions of 
the oxygen set free during the preparation, we wall carry it out of 
the way, and be prepared to make our experiments in a regular, 
dignified manner.) And inasmuch as that power of making wood, 
wax, or other things burn, was so marked in the oxygen we obtained 
by means of the voltaic battery from water, we may expect to find 
the same property here. We will try it. You see there is the com- 
bustion of a lighted taper in air, and here is its combustion in this 
gas [lowering the taper into the jar]. See how brightly and how 



CHEMICAL HISTORY OF A CANDLE I37 

beautifully it burns! You can also see more than this: you will per- 
ceive it is a heavy gas, while the hydrogen would go up like a 
balloon, or even faster than a balloon, when not encumbered with 
the weight of the envelope. You may easily see that although we 
obtained from water twice as much in volume of the hydrogen as of 
oxygen, it does not follow that we have twice as much in weight, 
because one is heavy and the other a very light gas. We have means 
of weighing gases or air; but, without stopping to explain that, let 
me just tell you what their respective weights are. The weight _ 
of a pint of hydrogen is three-quarters of a grain; the weight of ^ 
the same quantity of oxygen is nearly twelve grains. This is a 
very great difference. The weight of a cubic foot of hydrogen 
is one-twelfth of an ounce; and the weight of a cubic foot of 
oxygen is one ounce and a third. And so on we might come 
to masses of matter which may be weighed in the balance, and 
which we can take account of as to hundred-weights and as to 
tons, as you will see almost immediately. 

Now, as regards this very property of oxygen supporting 
combustion, which we may compare to air, I will take a piece 
of candle to show it you in a rough way — and the result will be 
rough. There is our candle burning in the air: how will it burn 
in oxygen ? I have here a jar of this gas, and I am about to put it over 
the candle for you to compare the action of this gas with that of the 
air. Why, look at it; it looks something like the light you saw at the 
poles of the voltaic battery. Think how vigorous that action must be. 
And yet, during all that action, nothing more is produced than what 
is produced by the burning of the candle in air. We have the same 
production of water, and the same phenomena exactly, when we use 
this gas instead of air, as we have when the candle is burnt in air. 

But now we have got a knowledge of this new substance, we can 
look at it a little more distinctly, in order to satisfy ourselves that 
we have got a good general understanding of this part of the product 
of a candle. It is wonderful how great the supporting powers of this 
substance are as regards combustion. For instance, here is a lamp 
which, simple though it be, is the original, I may say, of a great 
variety of lamps which are constructed for divers purposes — for 
lighthouses, microscopic illuminations, and other uses; and if it was 




Fig. 77 



138 FARADAY 

proposed to make it burn very brightly, you would say, "I£ a candle 
burnt better in oxygen, will not a lamp do the same?" Why, it will 
do so. Mr. Anderson will give me a tube coming from our oxygen 
reservoir, and I am about to apply it to this flame, which I will 
previously make burn badly on purpose. There comes the oxygen: 
what a combustion that makes! But if I shut it off, what becomes 
of the lamp? [The flow of oxygen was stopped, and the lamp 
relapsed to its former dimness.] It is wonderful how, 
by means of oxygen, we get combustion accelerated. But 
it does not affect merely the combustion of hydrogen, or 
carbon, or the candle, but it exalts all combustions of the 
common kind. We will take one which relates to iron, for 
instance, as you have already seen iron burn a little in the 
atmosphere. Here is a jar of oxygen, and this is a piece 
of iron wire; but if it were a bar as thick as my wrist, 
it would burn the same. I first attach a little piece of 
wood to the iron; I then set the wood on fire, and let them both 
down together into the jar (Fig. 77). The wood is now alight, and 
there it burns as wood should burn in oxygen; but it will soon com- 
municate its combustion to the iron. The iron is now burning 
brilliantly, and will continue so for a long time. As long as we 
supply oxygen, so long can we carry on the combustion 
of the iron, until the latter is consumed. 

We will now put that on one side, and take some 
other substance; but we must limit our experiments, for 
we have not time to spare for all the illustrations you 
would have a right to if we had more time. We will 
take a piece of sulphur: you know how sulphur burns in 
the air; well, we put it into the oxygen, and you will see 
that whatever can burn in air can burn with a far 
greater intensity in oxygen, leading you to think that 
perhaps the atmosphere itself owes all its power of com- 
bustion to this gas. The sulphur is now burning very quietly in the 
oxygen; but you can not for a moment mistake the very high and 
increased action which takes place when it is so burnt, instead of 
being burnt merely in common air. 

I am now about to show you the combustion of another sub- 




FlG. 78 



CHEMICAL HISTORY OF A CANDLE 1 39 

Stance — phosphorus. I can do it better for you here than you can do 
it at home. This is a very combustible substance; and if it be so 
combustible in air, what might you expect it would be in oxygen? 
I am about to show it to you not in its fullest intensity, for if I did 
so we should almost blow the apparatus up; I may even now crack 
the jar, though I do not want to break things carelessly. You see 
how it burns in the air. But what a glorious light it gives out when 
I introduce it into oxygen! [Introducing the lighted phosphorus 
into the jar of oxygen.] There you see the solid particles going off 
which cause that combustion to be so brilliantly luminous. 

Thus far we have tested this power of oxygen, and the high com- 
bustion it produces, by means of other substances. We must now, 
for a little while longer, look at it as respects the hydrogen. You 
know, when we allowed the oxygen and the hydrogen derived from 
the water to mix and burn together, we had a little explosion. You 
remember also that when I burnt the oxygen and the hydrogen in a 
jet together, we got very little light, but great heat; I am now about 
to set fire to oxygen and hydrogen mixed in the proportion in 
which they occur in water. Here is a vessel containing one volume of 
oxygen and two volumes of hydrogen. This mixture is exactly of 
the same nature as the gas we just now obtained from the voltaic 
battery; it would be far too much to burn at once; I have therefore 
arranged to blow soap bubbles with it and burn those bubbles, that 
we may see by a general experiment or two how this oxygen sup- 
ports the combustion of the hydrogen. First of all, we will see 
whether we can blow a bubble. Well, there goes the gas [causing 
it to issue through a tobacco-pipe into some soapsuds]. Here I have 
a bubble. I am receiving them on my hand, and you will perhaps 
think I am acting oddly in this experiment, but it is to show you 
that we must not always trust to noise and sounds, but rather to 
real facts. [Exploding a bubble on the palm of his hand.] I am 
afraid to fire a bubble from the end of the pipe, because the explo- 
sion would pass up into the jar and blow it to pieces. This oxygen, 
then, will unite with the hydrogen, as you see by the phenomena, 
and hear by the sound, with the utmost readiness of action, and all 
its powers are then taken up in its neutralization of the qualities of 
the hydrogen. 



140 FARADAY 

So now I think you will perceive the whole history of water with 
reference to oxygen and the air from what we have before said. 
Why does a piece of potassium decompose water? Because it finds 
oxygen in the water. What is set free when I put it in the water, as 
I am about to do again? It sets free hydrogen, and the hydrogen 
burns; but the potassium itself combines with oxygen; and this 
piece of potassium, in taking the water apart — the water, you may 
say, derived from the combustion of the candle — takes away the 
oxygen which the candle took from the air, and so sets the hydrogen 
free; and even if I take a piece of ice, and put a piece of potassium 
upon it, the beautiful affinities by which the oxygen and the hydro- 
gen are related are such that the ice will absolutely set fire to the 
potassium. I show this to you to-day, in order to enlarge your ideas 
of these things, and that you may see how greatly results are modified 
by circumstances. There is the potassium on the ice, producing a sort 
of volcanic action. 

It will be my place when next we meet, having pointed out these 
anomalous actions, to show you that none of these extra and strange 
effects are met with by us — that none of these strange and injurious 
actions take place when we are burning, not merely a candle, but 
gas in our streets, or fuel in our fireplaces, so long as we confine 
ourselves within the laws that Nature has made for our guidance. 



LECTURE V 

OXYGEN PRESENT IN THE AIR— NATURE OF THE 
ATMOSPHERE— ITS PROPERTIES— OTHER PROD- 
UCTS FROM THE CANDLE— CARBONIC 
ACID— ITS PROPERTIES 

WE have now seen that we can produce hydrogen and 
oxygen from the water that we obtained from the candle. 
Hydrogen, you know, comes from the candle, and oxygen, 
you believe, comes from the air. But then you have a right to ask me, 
"How is it that the air and the oxygen do not equally well bum' 
the candle?" If you remember what happened when I put a jar of 
oxygen over a piece of candle, you recollect there was a very dif- 
ferent kind of combustion to that which took place in the air. Now, 
why is this? It is a very important question, and one I shall endeavor 
to make you understand; it relates most intimately to the nature of 
the atmosphere, and is most important to us. 

We have several tests for oxygen besides the mere burning of 
bodies; you have seen a candle burnt in oxygen or in the air; you 
have seen phosphorus burnt in the air or in oxygen, and you have 
seen iron filings burnt in oxygen. But we have other tests besides 
these, and I am about to refer to one or two of them for the purpose 
of carrying your conviction and your experience farther. Here we 
have a vessel of oxygen. I will show its presence to you: if I take a 
litde spark and put it into that oxygen, you know by the experience 
you gained the last time we met what will happen — ^if I put that 
spark into the jar, it will tell you whether we have oxygen here or 
not. Yes! We have proved it by combustion; and now here is an- 
other test for oxygen, which is a very curious and useful one. I have 
here two jars full of gas, with a plate between them to prevent their 
mixing; I take the plate away, and the gases are creeping one into 
the other. "What happens?" say you; "they together produce no 
such combustion as was seen in the case of the candle." But see how 

141 



142 FARADAY 

the presence of oxygen is told by its association with this other sub- 
stance.('°) What a beautifully colored gas I have obtained in this 
way, showing me the presence of the oxygen! In the same way we 
can try this experiment by mixing common air with this test-gas. 
Here is a jar containing air — such air as the candle would burn in — 
and here is a jar or bottle containing the test-gas. I let them come 
together over water, and you see the result: the contents of the test- 
bottle are flowing into the jar of air, and you see I obtain exactly 
the same kind of action as before, and that shows me that there is 
oxygen in the air — the very same substance that has been already 
obtained by us from the water produced by the candle. But then, 
beyond that, how is it that the candle does not burn in air as well 
as in oxygen ? We will come to that point at once. I have here two 
jars; they are filled to the same height with gas, and the appearance 
to the eye is alike in both, and I really do not know at present which 
of these jars contains oxygen and which contains air, although I 
know they have previously been filled with these gases. But here 
is our test-gas, and I am going to work with the two jars, in order to 
examine whether there is any difference between them in the quality 
of reddening this gas. I am now going to turn this test-gas into one 
of the jars, and observe what happens. There is reddening, you see; 
there is, then, oxygen present. We will now test the other jar; 
but you see this is not so distinctly red as the first, and, farther, 
this curious thing happens: if I take these two gases and shake them 
well together with water, we shall absorb the red gas; and then, if I 
put in more of this test-gas and shake again, we shall absorb more; 
and I can go on as long as there be any oxygen present to produce 
that effect. If I let in air, it will not matter; but the moment I intro- 
duce water, the red gas disappears; and I may go on in this way, put- 
ting in more and more of the test-gas, until I come to something left 
behind which will not redden any longer by the use of that particular 
body that rendered the air and the oxygen red. Why is that? You 
see in a moment it is because there is, besides oxygen, something else 
present which is left behind. I will let a little more air into the jar, 
and if it turns red you will know that some of that reddening gas 

'^ The gas which is thus employed as a test for the presence of oxygen is the 
binoxide of nitrogen, or nitrous oxide. It is a colorless gas, which, when brought in 
contact with oxygen, unites with it, forming hyponitric acid, the red gas referred to. 



CHEMICAL HISTORY OF A CANDLE 1 43 

is still present, and that, consequently, it was not for the want of 
this producing body that that air was left behind. 

Now you will begin to understand what I am about to say. You 
saw that when I burnt phosphorus in a jar, as the smoke produced 
by the phosphorus and the oxygen of the air condensed, it left a good 
deal of gas unburnt, just as this red gas left something untouched; 
there was, in fact, this gas left behind, which the phosphorus can not 
touch, which the reddening gas can not touch, and this something is 
not oxygen, and yet is part of the atmosphere. 

So that is one way of opening out air into the two things of which 
it is composed — oxygen, which burns our candles, our phosphorus, 
or any thing else, and this other substance — nitrogen — which will not 
burn them. This other part of the air is by far the larger proportion, 
and it is a very curious body when we come to examine it; it is re- 
markably curious, and yet you say, perhaps, that it is very uninterest- 
ing. It is uninteresting in some respects because of this, that it shows 
no brilliant effects of combustion. If I test it with a taper as I do 
oxygen and hydrogen, it does not burn like hydrogen, nor does it 
make the taper burn like oxygen. Try it in any way I will, it does 
neither the one thing nor the other; it will not take fire; it will not let 
the taper burn; it puts out the combustion of every thing. There is 
nothing that will burn in it in common circumstances. It has no 
smell; it is not sour; it does not dissolve in water; it is neither an 
acid nor an alkali; it is as indifferent to all our organs as it is possible 
for a thing to be. And you might say, "It is nothing; it is not worth 
chemical attention; what does it do in the air.?" Ah! then come our 
beautiful and fine results shown us by an observant philosophy. Sup- 
pose, in place of having nitrogen, or nitrogen and oxygen, we had 
pure oxygen as our atmosphere; what would become of us? You 
know very well that a piece of iron lit in a jar of oxygen goes on burn- 
ing to the end. When you see a fire in an iron grate, imagine where 
the grate would go to if the whole of the atmosphere were oxygen. 
The grate would burn up more powerfully than the coals; for the 
iron of the grate itself is even more combustible than the coals which 
we burn in it. A fire put into the middle of a locomotive would be a 
fire in a magazine of fuel, if the atmosphere were oxygen. The ni- 
trogen lowers it down and makes it moderate and useful for us, 



144 FARADAY 

and then, with all that, it takes away with it the fumes that you have 
seen produced from the candle, disperses them throughout the whole 
of the atmosphere, and carries them away to places where they are 
wanted to perform a great and glorious purpose of good to man, for 
the sustenance of vegetation, and thus does a most wonderful work, 
although you say, on examining it, "Why, it is a perfectly indifferent 
thing." This nitrogen in its ordinary state is an inactive element; no 
action short of the most intense electric force, and then in the most 
infinitely small degree, can cause the nitrogen to combine directly 
with the other element of the atmosphere, or with other things round 
about it; it is a perfectly indifferent, and therefore to say, a safe sub- 
stance. 

But, before I take you to that result, I must tell you about the atmos- 
phere itself. I have written on this diagram the composition of one 
hundred parts of atmospheric air: 

Bulk. Weight. 

Oxygen 20 23.3 

Nitrogen 80 77.7 



It is a true analysis of the atmosphere so far as regards the quantity 
of oxygen and the quantity of nitrogen present. By our analysis, we 
find that 5 pints of the atmosphere contain only i pint of oxygen, 
and 4 pints, or 4 parts, of nitrogen by bulk. That is our analysis 
of the atmosphere. It requires all that quantity of nitrogen to reduce 
the oxygen down, so as to be able to supply the candle properly with 
fuel, so as to supply us with an atmosphere which our lungs can 
healthily and safely breathe; for it is just as important to make the 
oxygen right for us to breathe, as it is to make the atmosphere right 
for the burning of the fire and the candle. 

But now for this atmosphere. First of all, let me tell you the weight 
of these gases. A pint of nitrogen weighs 10 4-10 grains, or a cubic 
foot weighs i 1-6 ounces. That is the weight of the nitrogen. The 
oxygen is heavier: a pint of it weighs 11 9-10 grs., and a cubic foot 
weighs I 3-4 oz. A pint of air weighs about 10 7-10 grs., and a cubic 
foot I 1-5 oz. 

You have asked me several times, and I am very glad you have, 
"How do you weigh gases.?" I will show you: it is very simple, and 



CHEMICAL HISTORY OF A CANDLE 1 45 

easily done. Here is a balance, and here a copper bottle, made as light 
as we can consistent with due strength, turned very nicely in the 
lathe, and made perfectly air-tight, with a stop-cock, which we can 
open and shut, which at present is open, and, therefore, allows the 
bottle to be full of air. I have here a nicely-adjusted bal- 
ance in which I think the bottle, in its present condition, 
will be balanced by the weight on the other side. And 
here is a pump by which we can force the air into this 
bottle, and with it we will force in a certain number of 
volumes of air as measured by the pump. [Twenty meas- 
ures were pumped in.] We will shut that in and put it 
in the balance. See how it sinks; it is much heavier than 
it was. By what? By the air that we have forced into it 
by the pump. There is not a greater bull^^ of air, but there 
is the same bulk of heavier air, because we have forced 
in air upon it. And that you may have a fair notion in 
your mind as to how much this air measures, here is a 
jar full of water. We will open that copper vessel into 
this jar, and let the air return to its former -state. All I 
have to do now is to screw them tightly together, and to 
turn the taps, when there, you see, is the bulk of the 
twenty pumps of air which I forced into the bottle; and p,^ j^ 
to make sure that we have been quite correct in what we 
have been doing, we will take the bottle again to the balance, and, if it 
is now counterpoised by the original weight, we shall be quite sure we 
have made our experiment correctly. It is balanced; so, you see, we 
can find out the weight of the extra volumes of air forced in in that 
way, and by that means we are able to ascertain that a cubic foot of 
air weighs i 1-5 oz. But that small experiment will by no means 
convey to your mind the whole literal truth of this matter. It is won- 
derful how it accumulates when you come to larger volumes. This 
bulk of air [a cubic foot] weighs i 1-5 oz. What do you think of the 
contents of that box above there which I have had made for the pur- 
pose? The air which is within that box weighs one pound — a full 
pound; and I have calculated the weight of the air in this room: you 
would hardly imagine it, but it is above a ton. So rapidly do the 
weights rise up, and so important is the presence of the atmosphere, 



146 FARADAY 

and of the oxygen and the nitrogen in it, and the use it performs in 
conveying things to and fro from place to place, and carrying bad 
vapors to places where they will do good instead of harm. 

Having given you that little illustration with respect to the weight 
of the air, let me show you certain consequences of it. You have a 
right to them, because you would not un- 
derstand so much without it. Do you 
remember this kind of experiment? Have 
you ever seen it? Suppose I take a pump 
somewhat similar to the one I had a little 
while ago to force air into the bottle, and 
suppose I place it in such a manner that 
by certain arrangements I can apply my 
hand to it. My hand moves about in the 
air so easily that it seems to feel nothing, 
and I can hardly get velocity enough by 
any motion of my own in the atmosphere 
to make sure that there is much resistance 
to it. But when I put my hand here [on 
the air-pump receiver, which was afterward 
exhausted] you see what happens. Why is 
my hand fastened to this place, and why 
am I able to pull this pump about? And 
see! how is it that I can hardly get my hand 
away? Why is this? It is the weight of the air— the weight of the 
air that is above. I have another experiment here, which I think will 
explain to you more about it. When the air is pumped from under- 
neath the bladder which is stretched over this glass, you will see 
the effect in another shape: the top is quite flat at present, but I will 
make a very little motion with the pump, and now look at it; see 
how it has gone down, see how it is bent in; you will see the bladder 
go in more and more, until, at last, I expect it will be driven in and 
broken by the force of the atmosphere pressing upon it. [The bladder 
at last broke with a loud report.] Now that was done entirely by the 
weight of the air pressing on it, and you can easily understand how 
that is. The particles that are piled up in the atmosphere stand upon 
each other, as these five cubes do; you can easily conceive that four of 




Fig. 80 



CHEMICAL HISTORY OF A CANDLE 1 47 

these five cubes are resting upon the bottom one, and if I take that 
away the others will all sink down. So it is with the atmosphere; the 
air that is above is sustained by the air that is beneath, and when the 
air is pumped away from beneath them, the change occurs which you 




Fig. 81 

saw when I placed my hand on the air-pump, and which you saw in 
the case of the bladder, and which you shall see better here. I have tied 
over this jar a piece of sheet India-rubber, and I am now about to take 
away the air from the inside of the jar; and if you will watch the 
India-rubber — which acts as a partition between the air below and the 
air above — you will see, when I pump, how the pressure shows itself. 
See where it is going to: I can actually put 
my hand into the jar; and yet this result is 
only caused by the great and powerful action 
of the air above. How beautifully it shows 
this curious circumstance! 

Here is something that you can have a pull 
at when I have finished to-day. It is a little 
apparatus of two hollow brass hemispheres, 
closely fitted together, and having connected 
with it a pipe and a cock, through which we 
can exhaust the air from the inside; and 
although the two halves are so easily taken 
apart, while the air is left within, yet you will 
see, when we exhaust it by-and-by, no power of any two of you will 
be able to pull them apart. Every square inch of surface that is 
contained in the area of that vessel sustains fifteen pounds by weight, 
or nearly so, when the air is taken out, and you may try your strength 




Fig. 82 



148 FARADAY 

presently in seeing whether you can overcome that pressure of the 
atmosphere. 

Here is another very pretty thing — the boys' sucker, only refined 
by the philosopher. We young ones have a perfect right to take toys, 
and make them into philosophy, inasmuch as nowadays we are turn- 
ing philosophy into toys. Here is a sucker, only it is made of India- 
rubber. If I clap it upon the table, you see at once it holds. Why does 
it hold? I can slip it about, and yet if I try to pull it up, it seems as 
if it would pull the table with it. I can easily make it slip about 
from place to place, but only when I bring it to the edge of the table 
can I get it off. It is only kept down by the pressure of the at- 
mosphere above; we have a couple of them, and if you take these 
two and press them together, you will see how firmly they stick. 
And, indeed, we may use them as they are proposed to be used, to 
stick against windows, or against walls, where they will adhere for an 
evening, and serve to hang any thing on that you want. I think, how- 
ever, that you boys ought to be shown experiments that you can 
make at home; and so here is a very pretty experiment in illustration 
of the pressure of the atmosphere. Here is a tumbler of water. 
Suppose I were to ask you to turn that tumbler upside down so that 
the water should not fall out, and yet not be kept in by your hand, 
but merely by using the pressure of the atmosphere; could you do 
that? Take a wine-glass, either quite full or half full of water, and 
put a flat card on the top; turn it upside down, and then see what 
becomes of the card and of the water. The air can not get in because 
the water, by its capillary attraction round the edge, keeps it out. 

I think this will give you a correct notion of what you may call the 
materiality of the air; and when I tell you that that box holds a 
pound of it, and this room more than a ton, you will begin to think 
that air is something very serious. I will make another experiment to 
convince you of this positive resistance. There is that beautiful ex- 
periment of the popgun, made so well and so easily, you know, out 
of a quill, or a tube, or any thing of that kind, — where we take a slice 
of potato, for instance, or an apple, and take the tube and cut out 
a pellet, as I have now done, and push it to one end. I have made 
that end tight; and now I take another piece and put it in: it will 
confine the air that is within the tube perfectly and completely for 



CHEMICAL HISTORY OF A CANDLE 1 49 

our purpose; and I shall now find it absolutely impossible by any 
force of mine to drive that little pellet close up to the other. It 
can not be done; I may press the air to a certain extent, but if I 
go on pressing, long before it comes to the second, the confined air 
will drive the front one out with a force something like that of 
gunpowder; for gunpowder is in part dependent upon the same 
action that you see here exemplified. 

I saw the other day an experiment which pleased me much, as I 
thought it would serve our purpose here. (I ought to have held my 
tongue for four or five minutes before beginning this experiment, 
because it depends upon my lungs for success.) By the proper ap- 
plication of air, I expect to be able to drive this egg out of one cup 
into the other by the force of my breath; but if I fail it is in a good 
cause, and I do not promise success, because I have been talking more 
than I ought to do to make the experiment succeed. 

[The lecturer here tried the experiment, and succeeded in blowing 
the egg from one egg cup to the other.] 

You see that the air which I blow goes downward between the egg 
and the cup, and makes a blast under the egg, and is thus able to lift 
a heavy thing; for a full egg is a very heavy thing for air to lift. 
If you want to make the experiment, you had better boil the egg 
quite hard first, and then you may very safely try to blow it from 
one cup to the other, with a little care. 

I have now kept you long enough upon this property of the weight 
of the air, but there is another thing I should like to mention. You 
saw the way in which, in this popgun, I was able to drive the second 
piece of potato half or two-thirds of an inch before the first piece 
started, by virtue of the elasticity of the air, just as I pressed into the 
copper bottle the particles of air by means of the pump. Now this 
depends upon a wonderful property in the air, namely, its elasticity, 
and I should like to give you a good illustration of this. If I take any 
thing that confines the air properly, as this membrane, which also 
is able to contract and expand so as to give us a measure of the 
elasticity of the air, and confine in this bladder a certain portion of 
air; and then, if we take the atmosphere off from the outside of it, 
just as in these cases we put the pressure on — ^if we take the pressure 
off, you will see how it will then go on expanding and expanding, 



150 FARADAY 

larger and larger, until it will fill the whole of this bell jar, showing 
you that wonderful property of the air, its elasticity, its compressi- 
bility, and expansibility, to an exceedingly large extent, and which is 
very essential for the purposes and services it performs in the 
economy of creation. 

We will now turn to another very important part of our subject, 
remembering that we have examined the candle in its burning, and 
have found that it gives rise to various products. We have the prod- 
ucts, you know, of soot, of water, and of something else, which you 
have not yet examined. We have collected the water, but have 
allowed the other things to go into the air. Let us now examine 
some of these other products. 

Here is an experiment which I think will help you in part in this 
way. We will put our candle there, and place over it a chimney. 




thus. I think my candle will go on burning, because the air passage 
is open at the bottom and the top. In the first place, you see the 
moisture appearing — that you know about. It is water produced 
from the candle by the action of the air upon its hydrogen. But, 
besides that, something is going out at the top : it is not moisture — it is 
not water — it is not condensible; and yet, after all, it has very singu- 
lar properties. You will find that the air coming out of the top of 



CHEMICAL HISTORY OF A CANDLE I5I 

our chimney is nearly sufficient to blow the light out I am holding 
to it; and if I put the light fairly opposed to the current, it will blow 
it quite out. You will say, that is as it should be, and I am supposing 
that you think it ought to do so, because the nitrogen does not sup- 
port combustion, and ought to put the candle out, since the candle 
will not burn in nitrogen. But is there nothing else there than nitro- 
gen? I must now anticipate — ^that is to say, I must use my own 
knowledge to supply you with the means that we adopt for the pur- 
pose of ascertaining these things, and examining such gases as these. 
I will take an empty bottle — here is one — and if I hold it over this 
chimney, I shall get the combustion of the candle below sending its 
results into the bottle above; and we shall soon find that this bottle 
contains, not merely an air that is bad as regards the combustion of 
a taper put into it, but having other properties. 

Let me take a little quick-lime and pour some common water on 
to it — the commonest water will do. I will stir it a moment, then 
pour it upon a piece of filtering paper in a funnel, and we shall 
very quickly have a clear water proceeding to the bottle below, as I 
have here. I have plenty of this water in another bottle, but never- 
theless I should like to use the lime-water that was prepared before 
you, so that you may see what its uses are. If I take some of this 
beautiful clear lime-water, and pour it into this jar which has col- 
lected the air from the candle, you will see a change coming about. 
Do you see that the water has become quite milky? Observe, that 
will not happen wdth air merely. Here is a bottle filled with air; and 
if I put a little lime-water into it, neither the oxygen nor the nitro- 
gen, nor any thing else that is in that quantity of air, will make any 
change in the lime-water. It remains perfectly clear, and no shaking 
of that quantity of lime-water with that quantity of air in its com- 
mon state will cause any change; but if I take this bottle with the 
lime-water and hold it so as to get the general products of the 
candle in contact with it, in a very short time we shall have it milky. 
There is the chalk, consisting of the lime which we used in making 
the lime-water, combined with something that came from the candle 
— that other product which we are in search of, and which I want 
to tell you about today. This is a substance made visible to us by its 
action, which is not the action of the lime-water either upon the 



152 . FARADAY 

oxygen or upon the nitrogen, nor upon the water itself, but it is some- 
thing new to us from the candle. And then we find this white pow- 
der, produced by the lime-water and the vapor from the candle, ap- 
pears to us very much like whitening or chalk, and when examined 
it does prove to be exactly the same substance as whitening or chalk. 
So we are led, or have been led, to observe upon the various circum- 
stances of this experiment, and to trace this production of chalk 
to its various causes, to give us the true knowledge of the nature 
of this combustion of the candle — to find that this substance issuing 
from the candle is exactly the same as that substance which would 
issue from a retort if I were to put some chalk into it with a little 
moisture and make it red-hot; you would then find that exactly the 
same substance would issue from it as from the candle. 

But we have a better means of getting this substance, and in 
greater quantity, so as to ascertain what its general characters are. 
We find this substance in very great abundance in a multitude of 
cases where you would least expect it. All limestones contain a great 
deal of this gas which issues from the candle, and which we call 
carbonic acid. All chalks, all shells, all corals, contain a great quan- 
tity of this curious air. We find it fixed in these stones, for which 
reason Dr. Black called it "fixed air" — finding it in these fixed things 
like marble and chalk — he called it fixed air because it lost its 
quality of air, and assumed the condition of a solid body. We can 
easily get this air from marble. Here is a jar containing a little 
muriatic acid, and here is a taper which, if I put it into that jar, will 
show only the presence of common air. There is, you see, pure air 
down to the bottom; the jar is full of it. Here is a substance — marble 
("), a very beautiful and superior marble — and if I put these pieces 
of marble into the jar, a great boiling apparently goes on. That, 
however, is not steam; it is a gas that is rising up; and if I now 
search the jar by a candle, I shall have exactly the same effect pro- 
duced upon the taper as I had from the air which issued from the 
end of the chimney over the burning candle. It is exactly the same 
action, and caused by the very same substance that issued from the 
candle; and in this way we can get carbonic acid in great abundance 

^''Marble is a compound of carbonic acid and lime. The muriatic acid, being the 
stronger of the two, takes the place of the carbonic acid, which escapes as a gas, 
the residue forming muriate of lime or chloride of calcium. 



CHEMICAL HISTORY OF A CANDLE I53 

— we have already nearly filled the jar. We also find that this gas is 
not merely contained in marble. Here is a vessel in which I have 
put some common whitening — chalk which has been washed in 
water and deprived of its coarser particles, and so supplied to the 
plasterer as whitening — here is a large jar containing this whitening 
and water; and I have here some strong sulphuric acid, which is the 
acid you might have to use if you were to make these experiments 
(only, in using this acid with limestone, the body that is produced 
is an insoluble substance, whereas the muriatic acid produces a 
soluble substance that does not so much thicken the water). And 
you will seek out a reason why I take this kind of apparatus for 
the purpose of showing this experiment. I do it because you may 
repeat in a small way what I am about to do in a large one. You 
will have here just the same kind of action; and I am evolving in 
this large jar carbonic acid exactly the same in its nature and 
properties as the gas which we obtained from the combustion of the 
candle in the atmosphere. And, no matter how different the two 
methods by which we prepare this carbonic acid, you will see, when 
we get to the end of our subject, that it. is all exactly the same, 
whether prepared in the one way or in the other. 

We will now proceed to the next experiment with regard to this 
gas. What is its nature ? Here is one of the vessels full, and we will 
try it as we have done so many other gases — by combustion. You 
see it is not combustible, nor does it support combustion. Neither, as 
we know, does it dissolve much in water, because we collect it over 
water very easily. Then you know that it has an effect, and becomes 
white in contact with lime-water; and when it does become white 
in that way, it becomes one of the constituents to make carbonate 
of lime or limestone. 

The next thing I must show you is that it really does dissolve a 
little in water, and therefore that it is unlike oxygen and hydrogen 
in that respect. I have here an apparatus by which we can produce 
this solution. In the lower part of this apparatus is marble and acid, 
and in the upper part cold water. The valves are so arranged that 
the gas can get from one to the other. I will set it in action now, 
and you can see the gas bubbling up through the water, as it has been 
doing all night long, and by this time we shall find that we have 



Cubic Foot. 


^\ 


oz. 


li 


,J 


i^ 


»f 


I* 




ll»5 


1> 



154 FARADAY 

this substance dissolved in the water. If I take a glass and draw off 
some of the water, I find that it tastes a little acid to the mouth; it 
is impregnated with carbonic acid; and if I now apply a little lime- 
water to it, that will give us a test of its presence. This water will 
make the lime-water turbid and white, which is proof of the pres- 
ence of carbonic acid. 

Then it is a very weighty gas; it is heavier than the atmosphere. 
I have put their respective weights at the lower part of this table, 
along with, for comparison, the weights of the other gases we have 
been examining: 

Pint. 

Hydrogen | grs. 

Oxygen iit'it „ 

Nitrogen loA „ 

Air........ lofff „ 

Carbonic acid 16 J „ 

A pint of it weighs 16 1-3 grains, and a cubic foot weighs 1 9-10 
ounce, almost two ounces. You can see by many experiments that 
this is a heavy gas. Suppose I take a glass containing nothing else 
but air, and from this vessel containing the carbonic acid I attempt 
to pour a little of this gas into that glass — 
I wonder whether any has gone in or not. I 
can not tell by the appearance, but I can in 
this way [introducing the taper]. Yes, there 
it is, you see; and if I were to examine it by 
lime-water, I should find it by that test also. 
I will take this little bucket, and put it down 
into the well of carbonic acid — indeed, we 
too often have real wells of carbonic acid — 
and now, if there is any carbonic acid, I must 
have got to it by this time, and it will be in 
this bucket, which we will examine with a 
taper. There it is, you see; it is full of 
carbonic acid. 

There is another experiment by which I will show you its weight. 
I have here a jar suspended at one end of a balance — it is now 
equipoised; but when I pour this carbonic acid into the jar on the 
one side which now contains air, you will see it sink down at once 




Fig. 84 



CHEMICAL HISTORY OF A CANDLE 1 55 

because of the carbonic acid that I pour into it. And now, if I exam- 
ine this jar with the Hghted taper, I shall find that the carbonic 
acid has fallen into it, and it no longer has any power of supporting 
the combustion. If I blow a soap bubble, which of course will be 
filled with air, and let it fall into this jar of carbonic acid, it will 
float. But I shall first of all take one of these little balloons filled with 
air. I am not quite sure where the carbonic acid is; we will just try 




Fig. 85 

the depth, and see whereabouts is its level. There, you see, we 
have this bladder floating on the carbonic acid; and if I evolve some 
more of the carbonic acid, the bladder will be lifted up higher. 
There it goes; the jar is nearly full, and now I will see whether I 
can blow a soap bubble on that and float it in the same way. [The 
lecturer here blew a soap bubble and allowed it to fall into the 
jar of carbonic acid, when it floated in it midway.] It is floating, 
as the balloon floated, by virtue of the greater weight of the carbonic 
acid than of the air. And now, having so far given you the history 
of the carbonic acid, as to its sources in the candle, as to its physical 
properties and weight, when we next meet I shall show you of what 
it is composed, and where it gets its elements from. 



LECTURE VI 

CARBON OR CHARCOAL— COAL GAS— RESPIRATION 

AND ITS ANALOGY TO THE BURNING OF 

A CANDLE— CONCLUSION 

A LADY who honors me by her presence at these lectures has 
conferred a still farther obligation by sending me these two 
L. candles, which are from Japan, and, I presume, are made of 
that substance to which I referred in a former lecture. You see that 
they are even far more highly ornamented than the French candles, 
and, I suppose, are candles of luxury, judging from their appearance. 
They have a remarkable peculiarity about them, — namely, a hollow 
wick, — that beautiful peculiarity which Argand introduced into the 
lamp and made so valuable. To those who receive such presents from 
the East, I may just say that this and such hke materials gradually 
undergo a change which gives them on the surface a dull and dead 
appearance; but they may easily be restored to their original beauty 
if the surface be rubbed with a clean cloth or silk handkerchief, so 
as to polish the little rugosity or roughness: this will restore the 
beauty of the colors. I have so rubbed one of these candles, and you 
see the difference between it and the other which has not been 
polished, but which may be restored by the same process. Observe, 
also, that these moulded candles from Japan are made more conical 
than the moulded candles in this part of the world. 

I told you, when we last met, a good deal about carbonic acid. We 
found by the lime-water test that when the vapor from the top of 
the candle or lamp was received into bottles and tested by this solu- 
tion of lime-water (the composition of which I explained to you, and 
which you can make for yourselves), we had that white opacity 
which was in fact calcareous matter, like shells and corals, and many 
of the rocks and minerals in the earth. But I have not yet told you 
fully and clearly the chemical history of this substance, carbonic 
acid, as we have it from the candle, and I must now resume that 

156 



CHEMICAL HISTORY OF A CANDLE 1 57 

subject. We have seen the products, and the nature of them, as they 
issue from the candle. We have traced the water to its elements, and 
now we have to see where are the elements of the carbonic acid 
supplied by the candle. A few experiments will show this. You 
remember that when a candle burns badly it produces smoke; but 
if it is burning well there is no smoke. And you know that the 
brightness of the candle is due to this smoke, which becomes ignited. 
Here is an experiment to prove this: so long as the smoke remains in 
the flame of the candle and becomes ignited, it gives a beautiful 
light, and never appears to us in the form of black particles. I will 
light some fuel which is extravagant in its burning. This will serve 
our purpose — a little turpentine on a sponge. You see the smoke 
rising from it, and floating into the air in large quantities; and re- 
member now, the carbonic acid that we have from the candle is 
from such smoke as that. To make that evident to you, I will intro- 
duce this turpentine burning on the sponge into a flask where I have 
plenty of oxygen, the rich part of the atmosphere, and you now see 
that the smoke is all consumed. This is the first part of our experi- 
ment; and now, what follows? The carbon which you saw flying 
off from the turpentine flame in the air is now entirely burned in 
this oxygen, and we shall find that it will, by this rough and tem- 
porary experiment, give us exactly the same conclusion and result 
as we had from the combustion of the candle. The reason why I 
make the experiment in this manner is solely that I may cause 
the steps of our demonstration to be so simple that you can never 
for a moment lose the train of reasoning, if you only pay attention. 
All the carbon which is burned in oxygen, or air, comes out as 
carbonic acid, while those particles which are not so burned show 
you the second substance in the carbonic acid, — namely, the carbon — 
that body which made the flame so bright while there was plenty of 
air, but which was thrown off in excess when there was not oxygen 
enough to burn it. 

I have also to show you a little more distinctly the history of car- 
bon and oxygen in their union to make carbonic acid. You are now 
better able to understand this than before, and I have prepared three 
or four experiments by way of illustration. This jar is filled with 
oxygen, and here is some carbon which has been placed in a crucible 



158 FARADAY 

for the purpose of being made red-hot. I keep my jar dry, and ven- 
ture to give you a result imperfect in some degree, in order that I 
may make the experiment brighter. I am about to put the oxygen 
and the carbon together. That this is carbon (common charcoal 
pulverized) you will see by the way in which it burns in the air 
[letting some of the red-hot charcoal fall out of the crucible]. I am 
now about to burn it in oxygen gas, and look at the difference. It 
may appear to you at a distance as if it were burning with a flame; 
but it is not so. Every little piece of charcoal is burning as a spark, 
and while it so burns it is producing carbonic acid. I specially want 
these two or three experiments to point out what I shall dwell upon 
more distinctly by-and-by — that carbon burns in this way, and not 
as a flame. 

Instead of taking many particles of carbon to burn, I will take 
a rather large piece, which will enable you to see the form and size, 
and to trace the effects very decidedly. Here is the jar of oxygen, 
and here is the piece of charcoal, to which I have fastened a little 
piece of wood, which I can set fire to, and so commence the com- 
bustion, which I could not conveniently do without. You now see 
the charcoal burning, but not as a flame (or if there be a flame it is 
the smallest possible one, which I know the cause of, namely, the 
formation of a little carbonic oxide close upon the surface of the 
carbon). It goes on burning, you see, slowly producing carbonic 
acid by the union of this carbon or charcoal (they are equivalent 
terms) with the oxygen. I have here another piece of charcoal, a 
piece of bark, which has the quality of being blown to pieces — ex- 
ploding — as it burns. By the effect of the heat we shall reduce the 
lump of carbon into particles that will fly off; still every particle, 
equally with the whole mass, burns in this peculiar way — it burns as 
a coal and not like a flame. You observe a multitude of little com- 
bustions going on, but no flame. I do not know a finer experiment 
than this to show that carbon burns with a spark. 

Here, then, is carbonic acid formed from its elements. It is pro- 
duced at once; and if we examined it by lime-water you will see 
that we have the same substance which I have previously described 
to you. By putting together 6 parts of carbon by weight (whether 
it comes from the flame of a candle or from powdered charcoal) 



CHEMICAL HISTORY OF A CANDLE 1 59 

and 16 parts of oxygen by weight, we have 22 parts of carbonic acid; 
and, as we saw last time, the 22 parts of carbonic acid combined with 
28 parts of Hme, produced common carbonate of lime. If you were 
to examine an oyster-shell and weigh the component parts, you 
would find that every 50 parts would give 6 of carbon and 16 
of oxygen combined with 28 of lime. However, I do not want to 
trouble you with these minutiae; it is only the general philosophy 
of the matter that we can now go into. See how finely the carbon 
is dissolving away [pointing to the lump of charcoal burning quietly 
in the jar of oxygen]. You may say that the charcoal is actually dis- 
solving in the air round about; and if that were perfectly pure char- 
coal, which we can easily prepare, there would be no residue what- 
ever. When we have a perfectly cleansed and purified piece of car- 
bon, there is no ash left. The carbon burns as a solid dense body, 
that heat alone can not change as to its solidity, and yet it passes 
away into vapor that never condenses into solid or liquid under 
ordinary circumstances; and what is more curious still is the fact that 
the oxygen does not change in its bulk by the solution of the carbon 
in it. Just as the bulk is at first, so it is at last, only it has become 
carbonic acid. 

There is another experiment which I must give you before you 
are fully acquainted with the general nature of carbonic acid. Being 
a compound body, consisting of carbon and oxygen, carbonic acid 
is a body that we ought to be able to take asunder. And so we can. 
As we did with water, so we can with carbonic acid — take the two 
parts asunder. The simplest and quickest way is to act upon the 
carbonic acid by a substance that can attract the oxygen from it, and 
leave the carbon behind. You recollect that I took potassium and 
put it upon water or ice, and you saw that it could take the oxygen 
from the hydrogen. Now, suppose we do something of the same 
kind here with this carbonic acid. You know carbonic acid to be 
a heavy gas. I will not test it with lime-water, as that will interfere 
with our subsequent experiments, but I think the heaviness of the 
gas and the power of extinguishing flame will be sufficient for our 
purpose. I introduce a flame into the gas, and you will see whether it 
will be put out. You see the light is extinguished. Indeed, the gas 
may, perhaps, put out phosphorus, which you know has a pretty 



l6o FARADAY 

Strong combdstion. Here is a piece of phosphorus heated to a high 
degree. I introduce it into gas, and you observe the Hght is put out, 
but it will take fire again in the air, because there it re-enters into 
combustion. Now let me take a piece of potassium, a substance 
which even at common temperatures can act upon carbonic acid, 
though not sufficiently for our present purpose, because it soon gets 
covered with a protecting coat; but if we warm it up to the burning 
point in air, as we have a fair right to do, and as we have done with 
phosphorus, you will see that it can burn in carbonic acid; and if it 
burns, it will burn by taking oxygen, so that you will see what is left 
behind. I am going, then, to burn this potassium in the carbonic 
acid, as a proof of the existence of oxygen in the carbonic acid. [In 
the preliminary process of heating the potassium exploded.] Some- 
times we get an awkward piece of potassium that explodes, or some- 
thing like it, when it burns. I will take another piece, and now that 
it is heated I introduce it into the jar, and you perceive that it burns 
in the carbonic acid — not so well as in the air, because the carbonic 
acid contains the oxygen combined; but it does burn, and takes away 
the oxygen. If I now put this potassium into water, I find that 
besides the potash formed (which you need not trouble about) there 
is a quantity of carbon produced. I have here made the experiment 
in a very rough way, but I assure you that if I were to make it 
carefully, devoting a day to it instead of five minutes, we should get 
all the proper amount of charcoal left in the spoon, or in the place 
where the potassium was burned, so that there could be no doubt 
as to the result. Here, then, is the carbon obtained from the car- 
bonic acid, as a common black substance; so that you have the entire 
proof of the nature of carbonic acid as consisting of carbon and oxy- 
gen. And now I may tell you, that whenever carbon burns under 
common circumstances it produces carbonic acid. 

Suppose I take this piece of wood, and put it into a bottle with 
lime-water. I might shake that lime-water up with wood and the 
atmosphere as long as I pleased, it would still remain clear as 
you see it; but suppose I burn the piece of wood in the air of that 
bottle. You, of course, know I get water. Do I get carbonic acid? 
[The experiment was performed.] There it is, you see — that is to 
say, the carbonate of lime, which results from carbonic acid, and 



CHEMICAL HISTORY OF A CANDLE l6l 

that carbonic acid must be formed from the carbon which comes 
from the wood, from the candle, or any other thing. Indeed, you 
have yourselves frequently tried a very pretty experiment, by which 
you may see the carbon in wood. If you take a piece of wood, and 
partly burn it, and then blow it out, you have carbon left. There 
are things that do not show carbon in this way. A candle does not 
so show it, but it contains carbon. Here also is a jar of coal gas, 
which produces carbonic acid abundantly; you do not see the carbon, 
but we can soon show it to you. I will light it, and as long as there 
is any gas in the cylinder it will go on burning. You see no carbon, 
but you see a flame, and because that is bright it will lead you to guess 
that there is carbon in the flame. But I will show it to you by 
another process. I have some of the same gas in another vessel, mixed 
with a body that will burn the hydrogen of the gas, but will not burn 
the carbon. I will light them with a burning taper, and you perceive 
the hydrogen is consumed, but not the carbon, which is left behind as 
a dense black smoke. I hope that by these three or four experiments 
you will learn to see when carbon is present, and understand what 
are the products of combustion when gas or other bodies are thor- 
oughly burned in the air. 

Before we leave the subject of carbon, let us make a few experi- 
ments and remarks upon its wonderful condition as respects ordinary 
combustion. I have shown you that the carbon, in burning, burns 
only as a solid body, and yet you perceive that, after it is burned, 
it ceases to be a solid. There are very few fuels that act like this. It 
is, in fact, only that great source of fuel, the carbonaceous series, the 
coals, charcoals, and woods, that can do it. I do not know that there 
is any other elementary substance besides carbon that burns with 
these conditions; and if it had not been so, what would happen to us.'' 
Suppose all fuel had been like iron, which, when it burns, burns into 
a solid substance. We could not then have such a combustion as you 
have in this fireplace. Here also is another kind of fuel which burns 
very well — as well as, if not better, than carbon — so well, indeed, as 
to take fire of itself when it is in the air, as you see [breaking a tube 
full of lead pyrophorus]. This substance is lead, and you see how 
wonderfully combustible it is. It is very much divided, and is like a 
heap of coals in the fireplace; the air can get to its surface and inside. 



I 62 FARADAY 

and so it burns. But why does it not burn in that way now, when it 
is lying in a mass ? [emptying the contents of the tube in a heap on 
to a plate of iron.] Simply because the air can not get to it. Though 
it can produce a great heat, the great heat which we want in our 
furnaces and under our boilers, still that which is produced can not 
get away from the portion which remains unburned underneath, and 
that portion, therefore, is prevented from coming in contact with 
the atmosphere, and can not be consumed. How different is that 
from carbon! Carbon burns just in the same way as this lead does, 
and so gives an intense fire in the furnace, or wherever you choose 
to burn it; but then the body produced by its combustion passes 
away, and the remaining carbon is left clear. I showed you how 
carbon went on dissolving in the oxygen, leaving no ash, whereas 
here [pointing to the heap of pyrophorus] we have actually more ash 
than fuel, for it is heavier by the amount of the oxygen which has 
united with it. Thus you see the difference between carbon and lead 
or iron — if we choose iron, which gives so wonderful a result in our 
applications of this fuel, either as light or heat. If, when the carbon 
burnt, the product went off as a solid body, you would have had the 
room filled with an opaque substance, as in the case of the phos- 
phorus; but when carbon burns, every thing passes up into the 
atmosphere. It is in a fixed, almost unchangeable condition before 
the combustion; but afterward it is in the form of gas, which it is 
very difficult (though we have succeeded) to produce in a solid or 
liquid state. 

Now I must take you to a very interesting part of our subject — 
to the relation between the combustion of a candle and that living 
kind of combustion which goes on within us. In every one of us 
there is a living process of combustion going on very similar to that 
of a candle, and I must try to make that plain to you. For it is not 
merely true in a poetical sense — ^the relation of the life of man to a 
taper; and if you will follow, I think I can make this clear. In order 
to make the relation very plain, I have devised a little apparatus 
which we can soon build up before you. Here is a board, and a 
groove cut in it, and I can close the groove at the top part by a 
little cover; I can then continue the groove as a channel by a glass 
tube at each end, there being a free passage through the whole. 



CHEMICAL HISTORY OF A CANDLE 1 63 

Suppose I take a taper or candle (we can now be liberal in our use 
of the word "candle," since we understand what it means), and place 
it in one of the tubes; it will go on, you see, burning very well. 
You observe that the air which feeds the flame passes down the tube 
at one end, then goes along the horizontal tube, and ascends the tube 
at the other end in which the taper is placed. If I stop the aperture 
through which the air enters, I stop combustion, as you perceive. 
I stop the supply of air, and consequently the candle goes out. But 
now what will you think of this fact? In a former experiment I 




Fig. 86 

showed you the air going from one burning candle to a second 
candle. If I took the air proceeding from another candle, and sent 
it down by a complicated arrangement into this tube, I should put 
this burning candle out. But what will you say when I tell you that 
my breath will put out that candle ? I do not mean by blowing at all, 
but simply that the nature of my breath is such that a candle can 
not burn in it. I will now hold my mouth over the aperture, and 
without blowing the flame in any way, let no air enter the tube but 
what comes from my mouth. You see the result. I did not blow 
the candle out. I merely let the air which I expired pass into the 
aperture, and the result was that the light went out for want of 
oxygen, and for no other reason. Something or other — namely, my 
lungs — had taken away the oxygen from the air and there was no 
more to supply the combustion of the candle. It is, I think, very 
pretty to see the time it takes before the bad air which I throw into 
this part of the apparatus has reached the candle. The candle at first 
goes on burning, but so soon as the air has had time to reach it, it 



164 FARADAY 

goes out. And now I will show you another experiment, because this 
is an important part of our philosophy. Here is a jar which contains 
fresh air, as you can see by the circumstance of a candle or gas light 
burning it. I make it close for a little time, and by means of a pipe 
I get my mouth over it so that I can inhale the air. By putting it 
over water, in the way that you see, I am able to draw up this air 
(supposing the cork to be quite tight), take it 
into my lungs, and throw it back into the jar: 
we can then examine it, and see the result. You 
observe, I first take up the air, and then throw 
it back, as is evident from the ascent and de- 
scent of the water; and now, by putting a taper 
into the air, you will see the state in which it is, 
by the light being extinguished. Even one in- 
spiration, you see, has completely spoiled this 
air, so that it is no use my trying to breathe it 
a second time. Now you understand the ground 
of the impropriety of many of the arrangements 
among the houses of the poorer classes, by 
which the air is breathed over and over again 
for the want of a supply, by means of proper 
ventilation, sufficient to produce a good result. 
You see how bad the air becomes by a single breathing, so that 
you can easily understand how essential fresh air is to us. 

To pursue this a little farther, let us see what will happen with 
lime-water. Here is a globe which contains a little lime-water, and 
it is so arranged as regards the pipes, as to give access to the air 
within, so that we can ascertain the effect of respired or unrespired 
air upon it. Of course I can either draw in air (through A), and 
so make the air that feeds my lungs go through the lime-water, or 
I can force the air out of my lungs through the tube (B), which 
goes to the bottom, and so show its effect upon the lime-water. You 
will observe that however long I draw the external air into the lime- 
water and then through it to my lungs, I shall produce no effect 
upon the water — it will not make the lime-water turbid; but if I 
throw the air from my lungs through the lime-water several times 
in succession, you see how white and milky the water is getting, 




Fig. 87 




CHEMICAL HISTORY OF A CANDLE 1 65 

showing the effect which expired air has had upon it; and now you 
begin to know that the atmosphere which we have spoiled by respira- 
tion is spoiled by carbonic acid, for you see it here 
in contact with the lime-water. 

I have here two bottles, one containing lime- 
water and the other common water, and tubes 
which pass into the bottles and connect them. The 
apparatus is very rough, but it is useful notwith- 
standing. If I take these two bottles, inhaling here 
and exhaling there, the arrangement of the tubes 
will prevent the air going backward. The air 
coming in will go to my mouth and lungs, and 
in going out will pass through the lime-water, 
so that I can go on breathing, and making an 
experiment very refined in its nature and very good in its re- 
sults. You will observe that the good air has done nothing to the 
lime-water; in the other case, nothing has come to the lime-water 
but my respiration, and you see the difference in the two cases. 

Let us now go a little farther. What 
is all this process going on within us 
which we can not do without, either 
day or night, which is so provided for 
by the Author of all things that He has 
arranged that it shall be independent 
of all will.'' If we restrain our respira- 
tion, as we can to a certain extent, we 
should destroy ourselves. When we 
are asleep the organs of respiration and 
the parts that are associated with them 
still go on with their action, so neces- 
sary is this process of respiration to us, 
this contact of the air with the lungs. I 
must tell you, in the briefest possible 
manner, what this process is. We consume food; the food goes 
through that strange set of vessels and organs within us, and is 
brought into various parts of the system, into the digestive parts 
especially; and alternately the portion which is so changed is 




Fig. 89 



I 66 FARADAY 

carried through our lungs by one set of vessels, while the air that 
we inhale and exhale is drawn into and thrown out of the lungs 
by another set of vessels, so that the air and the food come close 
together, separated only by an exceedingly thin surface: the air 
can thus act upon the blood by this process, producing precisely 
the same results in kind as we have seen in the case of the candle. 
The candle combines with parts of the air, forming carbonic acid, 
and evolves heat; so in the lungs there is this curious, wonderful 
change taking place. The air entering, combines with the carbon 
(not carbon in a free state, but, as in this case, placed ready for 
action at the moment), and makes carbonic acid and is so thrown out 
into the atmosphere, and thus this singular result takes place; we 
may thus look upon the food as fuel. Let me take that piece of 
sugar, which will serve my purpose. It is a compound of carbon, 
hydrogen, and oxygen, similar to a candle, as containing the same 
elements, though not in the same proportion, the proportions being 
as shown in this table: 

SUGAK. 

Carbon 72 

Hydrogen " I 00 

Oxygen 88 M' 

This is, indeed, a very curious thing, which you can well remem- 
ber, for the oxygen and hydrogen are in exactly the proportions 
which form water, so that sugar may be said to be compounded of 
72 parts of carbon and 99 parts of water; and it is the carbon in 
the sugar that combines with the oxygen carried in by the air in 
the process of respiration — so making us like candles — producing 
these actions, warmth, and far more wonderful results besides, for 
the sustenance of the system, by a most beautiful and simple process. 
To make this still more striking, I will take a little sugar; or, to 
hasten the experiment, I will use some sirup, which contains about 
three-fourths of sugar and a little water. If I put a little oil of vitriol 
on it, it takes away the water, and leaves the carbon in a black mass. 
[The lecturer mixed the two together.] You see how the carbon is 
coming out, and before long we shall have a solid mass of charcoal, 
all of which has come out of sugar. Sugar, as you know, is food, and 
here we have absolutely a solid lump of carbon where you would not 



CHEMICAL HISTORY OF A CANDLE 1 67 

have expected it. And if I make arrangements so as to oxidize the 
carbon of sugar we shall have a much more striking result. Here is 
sugar, and I have here an oxidizer — a quicker one than the at- 
mosphere; and so we shall oxidize this fuel by a process different 
from respiration in its form, though not different in its kind. It is 
the combustion of the carbon by the contact of oxygen which the 
body has supplied to it. If I set this into action at once, you will see 
combustion produced. Just what occurs in my lungs — taking in oxy- 
gen from another source, namely, the atmosphere — takes place here 
by a more rapid process. 

You will be astonished when I tell you what this curious play of 
carbon amounts to. A candle will burn some four, five, six, or seven 
hours. What, then, must be the daily amount of carbon going up 
into the air in the way of carbonic acid! What a quantity of carbon 
must go from each of us in respiration! What a wonderful change 
of carbon must take place under these circumstances of combustion 
or respiration! A man in twenty-four hours converts as much as 
seven ounces of carbon into carbonic acid; a milch cow will convert 
seventy ounces, and a horse seventy-nine ounces, solely by the act 
of respiration. That is, the horse in twenty-four hours burns seventy- 
nine ounces of charcoal, or carbon, in his organs of respiration to 
supply his natural warmth in that time. All the warm-blooded 
animals get their warmth in this way, by the conversion of carbon, 
not in a free state, but in a state of combination. And what an 
extraordinary notion this gives us of the alterations going on in 
our atmosphere. As much as 5,000,000 pounds, or 548 tons, of 
carbonic acid is formed by respiration in London alone in twenty- 
four hours. And where does all this go? Up into the air. If the 
carbon had been like the lead which I showed you, or the iron which, 
in burning, produces a solid substance, what would happen? Com- 
bustion could not go on. As charcoal burns it becomes a vapor and 
passes off into the atmosphere, which is the great vehicle, the great 
carrier for conveying it away to other places. Then what becomes 
of it? Wonderful is it to find that the change produced by respira- 
tion, which seems so injurious to us (for we can not breathe air twice 
over), is the very life and support of plants and vegetables that grow 
upon the surface of the earth. It is the same also under the surface, 



I 68 FARADAY 

in the great bodies of water; for fishes and other animals respire 
upon the same principle, though not exactly by contact with the 
open air. 

Such fish as I have here [pointing to a globe of goldfish] respire 
by the oxygen which is dissolved from the air by the water, and form 
carbonic acid, and they all move about to produce the one great 
work of making the animal and vegetable kingdoms subservient to 
each other. And all the plants growing upon the surface of the 
earth, like that which I have brought here to serve as an illustration, 
absorb carbon; these leaves are taking up their carbon from the at- 
mosphere to which we have given it in the form of carbonic acid, 
and they are growing and prospering. Give them a pure air like 
ours, and they could not live in it; give them carbon with other 
matters, and they live and rejoice. This piece of wood gets all its 
carbon, as the trees and plants get theirs, from the atmosphere, 
which, as we have seen, carries away what is bad for us and at the 
same time good for them — ^what is disease to the one being health 
to the other. So are we made dependent not merely upon our fellow- 
creatures, but upon our fellow-existers, all Nature being tied to- 
gether by the laws that make one part conduce to the good of 
another. 

There is another little point which I must mention before we draw 
to a close— a point which concerns the whole of these operations, 
and most curious and beautiful it is to see it clustering upon and 
associated with the bodies that concern us — oxygen, hydrogen, and 
carbon, in different states of their existence. I showed you just now 
some powdered lead, which I set burning ('*) ; and you saw that the 
moment the fuel was brought to the air, it acted, even before it 
got out of the bottle — the moment the air crept in, it acted. Now 
there is a case of chemical affinity by which all our operations pro- 
ceed. When we breathe, the same operation is going on within us. 
When we burn a candle, the attraction of the different parts one to 
the other is going on. Here it is going on in this case of the lead, and 
it is a beautiful instance of chemical affinity. If the products of com- 

" Lead pyrophorus is made by heating dry tartrate of lead in a glass tube (closed at 
one end, and drawn out to a fine point at the other) until no more vapors are evolved. 
The open end of the tube is then to be sealed before the blowpipe. When the tube 
is broken and the contents shaken out into the air, they burn with a red flash. 



CHEMICAL HISTORY OF A CANDLE 1 69 

bustion rose off from the surface, the lead would take fire, and go 
on burning to the end; but you remember that we have this dif- 
ference between charcoal and lead — that, while the lead can start 
into action at once if there be access of air to it, the carbon will 
remain days, weeks, months, or years. The manuscripts of Her- 
culaneum were written with carbonaceous ink, and there they have 
been for 1,800 years or more, not having been at all changed by the 
atmosphere, though coming in contact with it under various circum- 
stances. Now, what is the circumstance which makes the lead and 
carbon differ in this respect.'' It is a striking thing to see that the 
matter which is appointed to serve the purpose of fuel waits in its 
action; it does not start off burning, like the lead and many other 
things that I could show you, but which I have not encumbered the 
table with; but it waits for action. This waiting is a curious and 
wonderful thing. Candles — those Japanese candles, for instance — do 
not start into action at once like the lead or iron (for iron finely 
divided does the same thing as lead), but there they wait for years, 
perhaps for ages, without undergoing any alteration. I have here a 
supply of coal gas. The jet is giving forth the gas, but you see it 
does not take fire — it comes out into the air, but it waits till it is 
hot enough before it burns. If I make it hot enough, it takes fire. 
If I blow it out, the gas that is issuing forth waits till the light is 
applied to it again. It is curious to see how different substances wait 
—how some will wait till the temperature is raised a little, and others 
till it is raised a good deal. I have here a httle gunpowder and some 
guncotton; even these things differ in the conditions under which 
they will burn. The gunpowder is composed of carbon and other 
substances, making it highly combustible; and the guncotton is 
another combustible preparation. They are both waiting, but they 
will start into activity at different degrees of heat, or under different 
conditions. By applying a heated wire to them, we shall see which, 
will start first [touching the guncotton with the hot iron]. You see 
the guncotton has gone off, but not even the hottest part of the 
wire is now hot enough to fire the gunpowder. How beautifully 
that shows you the difference in the degree in which bodies act in 
this way! In the one case the substance will wait any time until 
the associated bodies are made active by heat; but in the other, as 



170 FARADAY 

in the process of respiration, it waits no time. In the lungs, as soon 
as the air enters, it unites with the carbon; even in the lowest tem- 
perature which the body can bear short of being frozen, the action 
begins at once, producing the carbonic acid of respiration; and so 
all things go on fitly and properly. Thus you see the analogy between 
respiration and combustion is rendered still more beautiful and 
striking. Indeed, all I can say to you at the end of these lectures 
(for we must come to an end at one time or other) is to express a 
wish that you may, in your generation, be fit to compare to a candle; 
that you may, like it, shine as lights to those about you; that, in all 
your actions, you may justify the beauty of the taper by making 
your deeds honorable and effectual in the discharge of your duty 
to your fellow-men. 



ON THE CONSERVATION OF FORCE 
ICE AND GLACIERS 



BY 
HERMANN VON HELMHOLTZ 

TRANSLATED BY 
EDMUND ATKINSON 



INTRODUCTORY NOTE 

Hermann Ludwig Ferdinand von Helmholtz was born at Potsdam, 
near Berlin, on August 31, 1821. His father was a man of high culture, 
a teacher in the gymnasium, whose influence ensured to his son the 
foundations of a broad general education. His mother was a descendant 
from William Penn, the English Quaker. 

Helmholtz early showed mathematical ability, and wished to devote 
his life to the study of physics; but practical considerations led him to 
take up medicine, and he became a surgeon in the Prussian army. He 
began the publication of original contributions to science in 1842, and for 
fifty-two years, till his death in 1894, he continued to produce in an 
unbroken stream. He held a succession of academic positions, teaching 
physiology at Konigsberg, Bonn, and Heidelberg, and for the last twenty- 
three years of his life filling the chair of physics at Berlin. 

The tides of his professorships, however, give a very inadequate idea 
of his range. His contributions to science cover medicine, physiology, 
optics, acoustics, mathematics, mechanics, and electricity. His interests 
in science and art came together in his work on esthetics, and he had a 
lively appreciation of painting, poetry, and music. 

The practice of popular lecturing on scientific subjects was almost 
unknown in Germany when Helmholtz began, and he did much to give 
it dignity and to set a standard. His own lectures, as the reader of the 
following papers will perceive, are masterpieces of their kind. "The 
matter," says a biographer, "is discussed by a master, who brings to bear 
upon it all his wealth of learning and research, while there is the ever- 
enduring interest that attaches to an exposition by one who is giving 
forth from his own treasury." It is fortunate for the layman when a 
scientist and thinker of the first order has the skill and the inclination to 
share with the outside world the rich harvest of his brilliant and laborious 
research. 



ON THE CONSERVATION 
OF FORCE 

INTRODUCTION TO A SERIES OF LECTURES 

DELIVERED AT CARLSRUHE IN THE 

WINTER OF 1862-1863 

AS I have undertaken to deliver here a series of lectures, I think 
/ \ the best way in which I can discharge that duty will be 
A. jL, to bring before you, by means of a suitable example, some 
view of the special character of those sciences to the study of which 
I have devoted myself. The natural sciences, partly in consequence 
of their practical applications, and partly from their intellectual 
influence on the last four centuries, have so profoundly, and with 
such increasing rapidity, transformed all the relations of the life of 
civilised nations; they have given these nations such increase of 
riches, of enjoyment of life, of the preservation of health, of means 
of industrial and of social intercourse, and even such increase of 
political power, that every educated man who tries to understand the 
forces at work in the world in which he is living, even if he does 
not wish to enter upon the study of a special science, must have 
some interest in that peculiar kind of mental labour, which works 
and acts in the sciences in question. 

On a former occasion I have already discussed the characteristic 
differences which exist between the natural and the mental sciences 
as regards the kind of scientific work. I then endeavoured to show 
that it is more especially in the thorough conformity with law which 
natural phenomena and natural products exhibit, and in the com- 
parative ease with which laws can be stated, that this difference 
exists. Not that I wish by any means to deny, that the mental life 
of individuals and peoples is also in conformity with law, as is the 
object of philosophical, philological, historical, moral, and social 
sciences to establish. But in mental life, the influences are so inter- 

173 



174 HELMHOLTZ 

woven, that any definite sequence can but seldom be demonstrated 
In Nature the converse is the case. It has been possible to discover 
the law of the origin and progress of many enormously extended 
series of natural phenomena with such accuracy and completeness 
that we can predict their future occurrence with the greatest cer- 
tainty; or in cases in which we have power over the conditions under 
which they occur, we can direct them just according to our will. 
The greatest of all instances of what the human mind can effect by 
means of a well-recognised law of natural phenomena is that 
afforded by modern astronomy. The one simple law of gravitation 
regulates the motions of the heavenly bodies not only of our own 
planetary system, but also of the far more distant double stars; from 
which, even the ray of light, the quickest of all messengers, needs 
years to reach our eye; and, just on account of this simple con- 
formity with law, the motions of the bodies in question can be 
accurately predicted and determined both for the past and for future 
years and centuries to a fraction of a minute. 

On this exact conformity with law depends also the certainty with 
which we know how to tame the impetuous force of steam, and to 
make it the obedient servant of our wants. On this conformity 
depends, moreover, the intellectual fascination which chains the 
physicist to his subjects. It is an interest of quite a different kind to 
that which mental and moral sciences afford. In the latter it is man 
in the various phases of his intellectual activity who chains us. 
Every great deed of which history tells us, every mighty passion 
which art can represent, every picture of manners, of civic arrange- 
ments, of the culture of peoples of distant lands or of remote times, 
seizes and interests us, even if there is no exact scientific connection 
among them. We continually find points of contact and comparison 
in our conceptions and feelings; we get to know the hidden capac- 
ities and desires of the mind, which in the ordinary peaceful course 
of civilised life remain unawakened. 

It is not to be denied that, in the natural sciences, this kind of 
interest is wanting. Each individual fact, taken by itself, can indeed 
arouse our curiosity or our astonishment, or be useful to us in its 
practical applications. But intellectual satisfaction we obtain only 
from a connection of the whole, just from its conformity with law. 



ON THE CONSERVATION OF FORCE 1 75 

Reason we call that faculty innate in us of discovering laws and 
applying them with thought. For the unfolding of the peculiar 
forces of pure reason in their entire certainty and in their entire 
bearing, there is no more suitable arena than inquiry into Nature in 
the wider sense, the mathematics included. And it is not only the 
pleasure at the successful activity of one of our most essential mental 
powers; and the victorious subjections to the power of our thought 
and will of an external world, partly unfamiliar, and partly hostile, 
which is the reward of this labour; but there is a kind, I might almost 
say, of artistic satisfaction, when we are able to survey the enormous 
wealth of Nature as a regularly-ordered whole — a kosmos, an image 
of the logical thought of our own mind. 

The last decades of scientific development have led us to the 
recognition of a new universal law of all natural phenomena, which, 
from its extraordinarily extended range, and from the connection 
which it constitutes between natural phenomena of all kinds, even 
of the remotest times and the most distant places, is especially fitted 
to give us an idea of what I have described as the character of the 
natural sciences, which I have chosen as the subject of this lecture. 

This law is the Law of the Conservation of Force, a term the 
meaning of which I must first explain. It is not absolutely new; for 
individual domains of natural phenomena it was enunciated by 
Newton and Daniel Bernoulli; and Rumford and Humphry Davy 
have recognised distinct features of its presence in the laws of heat. 

The possibility that it was of universal application was first stated 
by Dr. Julius Robert Mayer, a Schwabian physician (now living in 
Heilbronn), in the year 1842, while almost simultaneously with, and 
independently of him, James Prescot Joule, an English manufacturer, 
made a series of important and difficult experiments on the rela- 
tion of heat to mechanical force, which supplied the chief points in 
which the comparison of the new theory with experience was still 
wanting. 

The law in question asserts, that the quantity of force which can 
be brought into action in the whole of Nature is unchangeable, and 
can neither be increased nor diminished. My first object will be to 
explain to you what is understood by quantity of force; or, as the 
same idea is more popularly expressed with reference to its technical 



176 HELMHOLTZ 

application, what we call amount of wor\ in the mechanical sense 
of the word. 

The idea of work for machines, or natural processes, is taken from 
comparison with the working power of man; and we can therefore 
best illustrate from human labour the most important features of 
the question with which we are concerned. In speaking of the work 
of machines and of natural forces we must, of course, in this com- 
parison eliminate anything in which activity of intelligence comes 
into play. The latter is also capable of the hard and intense work of 
thinking, which tries a man just as muscular exertion does. But 
whatever of the actions of intelligence is met with in the work of 
machines, of course is due to the mind of the constructor and cannot 
be assigned to the instrument at work. 

Now, the external work of man is of the most varied kind as 
regards the force or ease, the form and rapidity, of the motions used 
on it, and the kind of work produced. But both the arm of the black- 
smith who delivers his powerful blows with the heavy hammer, and 
that of the violinist who produces the most delicate variations in 
sound, and the hand of the lace-maker who works with threads so 
fine that they are on the- verge of the invisible, all these acquire the 
force which moves them in the same manner and by the same 
organs, namely, the muscles of the arms. An arm the muscles of 
which are lamed is incapable of doing any work; the moving force 
of the muscle must be at work in it, and these must obey the nerves, 
which bring to them orders from the brain. That member is then 
capable of the greatest variety of motions; it can compel the most 
varied instruments to execute the most diverse tasks. 

Just so it is with machines: they are used for the most diversified 
arrangements. We produce by their agency an infinite variety of 
movements, with the most various degrees of force and rapidity, 
from powerful steam hammers and rolling mills, where gigantic 
masses of iron are cut and shaped like butter, to spinning and 
weaving frames, the work of which rivals that of the spider. Modern 
mechanism has the richest choice of means of transferring the motion 
of one set of rolling wheels to another with greater or less velocity; 
of changing the rotating motion of wheels into the up-and-down 
motion of the piston rod, of the shuttle, of falling hammers and 



ON THE CONSERVATION OF FORCE 1 77 

Stamps; or, conversely, of changing the latter into the former; or it 
can, on the other hand, change movements of uniform into those of 
varying velocity, and so forth. Hence this extraordinarily rich 
utility of machines for so extremely varied branches of industry. 
But one thing is common to all these differences; they all need a 
moving force, which sets and keeps them in motion, just as the works 
of the human hand all need the moving force of the muscles. 

Now, the work of the smith requires a far greater and more 
intense exertion of the muscles than that of the violin player; and 
there are in machines corresponding differences in the power and 
duration of the moving force required. These differences, which 
correspond to the different degree of exertion of the muscles in 
human labour, are alone what we have to think of when we speak of 
the amount of tvorI{ of a machine. We have nothing to do here 
with the manifold character of the actions and arrangements which 
the machines produce; we are only concerned with an expenditure 
of force. 

This very expression which we use so fluently, 'expenditure of 
force,' which indicates that the force applied has been expended and 
lost, leads us to a further characteristic analogy between the effects 
of the human arm and those of machines. The greater the exer- 
tion, and the longer it lasts, the more is the. arm tired, and the more 
is the store of its moving force for the time exhausted. We shall see 
that this peculiarity of becoming exhausted by work is also met with 
in the moving forces of inorganic nature; indeed, that this capacity 
of the human arm of being tired is only one of the consequences of 
the law with which we are now concerned. When fatigue sets in, 
recovery is needed, and this can only be effected by rest and nourish- 
ment. We shall find that also in the inorganic moving forces, when 
their capacity for work is spent, there is a possibility of reproduction, 
although in general other means must be used to this end than in 
the case of the human arm. 

From the feeling of exertion and fatigue in our muscles, we can 
form a general idea of what we understand by amount of work; 
but we must endeavour, instead of the indefinite estimate afforded 
by this comparison, to form a clear and precise idea of the standard 
by which we have to measure the amount of work. This we can do 



178 HELMHOLTZ 

better by the simplest inorganic moving forces than by the actions 
of our muscles, which are a very complicated apparatus, acting in 
an extremely intricate manner. 

Let us now consider that moving force which we know best, and 
which is simplest — gravity. It acts, for example, as such in those 
clocks which are driven by a weight. This weight, fastened to a 
string, which is wound round a pulley connected with the first 
toothed wheel of the clock, cannot obey the pull of gravity without 
setting the whole clockwork in motion. Now I must beg you to 
pay special attention to the following points: the weight cannot 
put the clock in motion without itself sinking; did the weight not 
move, it could not move the clock, and its motion can only be such 
a one as obeys the action of gravity. Hence, if the clock is to go, the 
weight must continually sink lower and lower, and must at length 
sink so far that the string which supports it is run out. The clock 
then stops. The usual effect of its weight is for the present exhausted. 
Its gravity is not lost or diminished; it is attracted by the earth as 
before, but the capacity of this gravity to produce the motion of the 
clockwork is lost. It can only keep the weight at rest in the lowest 
point of its path, it cannot farther put it in motion. 

But we can wind up the clock by the power of the arm, by which 
the weight is again raised. When this has been done, it has regained 
its former capacity, and can again set the clock in motion. 

We learn from this that a raised weight possesses a moving force, 
but that it must necessarily sink if this force is to act; that by sink- 
ing, this moving force is exhausted, but by using another extraneous 
moving force — that of the arm — its activity can be restored. 

The work which the weight has to perform in driving the clock 
is not indeed great. It has continually to overcome the small resist- 
ances which the friction of the axles and teeth, as well as the 
resistance of the air, oppose to the motion of the wheels, and it has 
to furnish the force for the small impulses and sounds which the 
pendulum produces at each oscillation. If the weight is detached 
from the clock, the pendulum swings for a while before coming 
to a rest, but its motion becomes each moment feebler, and ultimately 
ceases entirely, being gradually used up by the small hindrances I 
have mentioned. Hence, to keep the clock going, there must be a 



ON THE CONSERVATION OF FORCE 1 79 

moving force, which, though small, must be continually at work. 
Such a one is the weight. 

We get, moreover, from this example, a measure for the amount of 
work. Let us assume that a clock is driven by a weight of a pound, 
which falls five feet in twenty-four hours. If we fix ten such clocks, 
each with a weight of one pound, then ten clocks will be driven 
twenty-four hours; hence, as each has to overcome the same resist- 
ances in the same time as the others, ten times as much work is 
performed for ten pounds fall through five feet. Hence, we con- 
clude that the height of the fall being the same, the work increases 
directly as the weight. 

Now, if we increase the length of the string so that the weight 
runs down ten feet, the clock will go two days instead of one; and, 
with double the height of fall, the weight will overcome on the 
second day the same resistances as on the first, and will therefore 
do twice as much work as when it can only run down five feet. The 
weight being the same, the work increases as the height of fall. 
Hence, we may take the product of the weight into the height of 
fall as a measure of work, at any rate, in the present case. The appli- 
cadon of this measure is, in fact, not limited to the individual case, 
but the universal standard adopted in manufactures for measuring 
magnitude of work is a foot pound — that is, the amount of work 
which a pound raised through a foot can produce.' 

We may apply this measure of work to all kinds of machines, 
for we should be able to set them all in motion by means of a weight 
sufficient to turn a pulley. We could thus always express the magni- 
tude of any driving force, for any given machine, by the magnitude 
and height of fall of such a weight as would be necessary to keep 
the machine going with its arrangements until it had performed a 
certain work. Hence it is that the measurement of work by foot 
pounds is universally applicable. The use of such a weight as a 
driving force would not indeed be practically advantageous in those 
cases in which we were compelled to raise it by the power of our 
own arm; it would in that case be simpler to work the machine by 
the direct action of the arm. In the clock we use a weight so that 

' This is the technical measure of work; to convert it into scientific measure it must 
be multiplied by the intensity of gravity. 



l8o HELMHOLTZ 

we need not stand the whole day at the clockwork, as we should 
have to do to move it directly. By winding up the clock we accumu- 
late a store of working capacity in it, which is sufficient for the 
expenditure of the next twenty-four hours. 

The case is somewhat different when Nature herself raises the 
weight, which then works for us. She does not do this with solid 




Fig. 90 



bodies, at least not with such regularity as to be utilised; but she 
does it abundantly with water, which, being raised to the tops of 
mountains by meteorological processes, returns in streams from them. 
The gravity of water we use as moving force, the most direct appli- 
cation being in what are called overshot wheels, one of which is 
represented in Fig. 90. Along the circumference of such a wheel are 
a series of buckets, which act as receptacles for the water, and, on 
the side turned to the observer, have the tops uppermost; on the 



ON THE CONSERVATION OF FORCE l8l 

opposite side the tops of the buckets are upside-down. The water 
flows at M into the buckets of the front of the wheel, and at F, 
where the mouth begins to incHne downwards, it flows out. The 
buckets on the circumference are filled on the side turned to the 
observer, and empty on the other side. Thus the former are weighted 
by the water contained in them, the latter not; the weight of the 
water acts continuously on only one side of the wheel, draws this 
down, and thereby turns the wheel; the other side of the wheel 
offers no resistance, for it contains no water. It is thus the weight 
of the falling water which turns the wheel, and furnishes the motive 
power. But you will at once see that the mass of water which turns 
the wheel must necessarily fall in order to do so, and that though, 
when it has reached the bottom, it has lost none of its gravity, it is 
no longer in a position to drive the wheel, if it is not restored to its 
original position, either by the power of the human arm or by means 
of some other natural force. If it can flow from the mill-stream to 
still lower levels, it may be used to work other wheels. But when 
it has reached its lowest level, the sea, the last remainder of the 
moving force is used up, which is due to gravity — that is, to the 
attraction of the earth, and it cannot act by its weight until it has 
been again raised to a high level. As this is actually effected by 
meteorological processes, you will at once observe that these are to 
be considered as sources of moving force. 

Water power was the first inorganic force which man learnt to 
use instead of his own labour or of that of domestic animals. 
According to Strabo, it was known to King Mithridates of Pontus, 
who was also otherwise celebrated for his knowledge of Nature; 
near his palace there was a water wheel. Its use was first introduced 
among the Romans in the time of the first Emperors. Even now we 
find water mills in all mountains, valleys, or wherever there are 
rapidly-flowing regularly-filled brooks and streams. We find water 
power used for all purposes which can possibly be effected by ma- 
chines. It drives mills which grind corn, sawmills, hammers and 
oil presses, spinning frames and looms, and so forth. It is the cheap- 
est of all motive powers, it flows spontaneously from the inexhaust- 
ible stores of Nature; but it is restricted to a particular place, and 
only in mountainous countries is it present in any quantity; in level 



1 82 HELMHOLTZ 

countries extensive reservoirs are necessary for damming the rivers 
to produce any amount of water power. 

Before passing to the discussion of other motive forces I must 
answer an objection which may readily suggest itself. We all know 
that there are numerous machines, systems of pulleys, levers and 
cranes, by the aid of which heavy burdens may be 
lifted by a comparatively small expenditure of 
force. We have all of us often seen one or two 
workmen hoist heavy masses of stones to great 
heights, which they would be quite unable to do 
directly; in like manner, one or two men, by 
means of a crane, can transfer the largest and 
heaviest chests from a ship to the quay. 
Now, it may be asked. If a large, heavy 
weight had been used for driving a machine, 
would it not be very easy, by means of a 
crane or a system of pulleys, to raise it anew, 
so that it could again be used as a motor, and thus 
acquire motive power, without being compelled to use 
a corresponding exertion in raising the weight? 

The answer to this is, that all these machines, in that 
degree in which for the moment they facilitate the exer- 
tion, also prolong it, so that by their help no motive 
power is ultimately gained. Let us assume that four 
labourers have to raise a load of four hundredweight 
by means of a rope passing over a single pulley. Every 
time the rope is pulled down through four feet, the 
load is also raised through four feet. But now, for the 
sake of comparison, let us suppose the same load hung 
to a block of four pulleys, as represented in Fig. 91. A 
single labourer would now be able to raise the load by 
the same exertion of force as each one of the four put 
forth. But when he pulls the rope through four feet, the 
load only rises one foot, for the length through which 
he pulls the rope, at a, is uniformly distributed in the 
block over four ropes, so that each of these is only shortened by a 
foot. To raise the load, therefore, to the same height, the one man 




Fig. 91 



ON THE CONSERVATION OF FORCE 1 83 

must necessarily work four times as long as the four together did. 
But the total expenditure of work is the same, whether four labour- 
ers work for a quarter of an hour or one works for an hour. 

If, instead of human labour, we introduce the work of a weight, 
and hang to the block a load of 400, and at a, where otherwise the 
labourer works, a weight of 100 pounds, the block is then in equi- 
librium, and, without any appreciable exertion of the arm, may be 
set in motion. The weight of 100 pounds sinks, that of 400 rises. 




Fig. 92 



Without any measurable expenditure of force, the heavy weight has 
been raised by the sinking of the smaller one. But observe that the 
smaller weight will have sunk through four times the distance that 
the greater one has risen. But a fall of 100 pounds through four 
feet is just as much 400 foot-pounds as a fall of 400 pounds through 
one foot. 

The action of levers in all their various modifications is precisely 
similar. Let a b, Fig. 92, be a simple lever, supported at c, the arm 
c b being four times as long as the other arm a c. Let a weight of 
one pound be hung at b, and a weight of four pounds at a, the lever 
is then in equilibrium, and the least pressure of the finger is sufficient, 
without any appreciable exertion of force, to place it in the position 
fl' b^, in which the heavy weight of four pounds has been raised, 
while the one-pound weight has sunk. But here, also, you will ob- 
serve no work has been gained, for while the heavy weight has been 



184 HELMHOLTZ 

raised through one inch, the lighter one has fallen through four 
inches; and four pounds through one inch is, as work, equivalent to 
the product of one pound through four inches. 

Most other fixed parts of machines may be regarded as modified 
and compound levers; a toothed-wheel, for instance as a series of 
levers, the ends of which are represented by the individual teeth, 
and one after the other of which is put in activity in the degree in 




Fig. 93 

which the tooth in question seizes or is seized by the adjacent pinion. 
Take, for instance, the crabwinch, represented in Fig. 93. Suppose 
the pinion on the axis of the barrel of the winch has twelve teeth, 
and the toothed-wheel, H H, seventy-two teeth, that is, six times as 
many as the former. The winch must now be turned round six 
times before the toothed-wheel, H, and the barrel, D, have made one 
turn, and before the rope which raises the load has been lifted by a 
length equal to the circumference of the barrel. The workman thus 
requires six times the time, though to be sure only one-sixth of the 
exertion, which he would have to use if the handle were directly 
applied to the barrel, D. In all these machines, and parts of ma- 



ON THE CONSERVATION OF FORCE 1 85 

chines, we find it confirmed that in proportion as the velocity of the 
motion increases its power diminishes, and that when the power 
increases the velocity diminishes, but that the amount of work is 
never thereby increased. 

In the overshot mill wheel, described above, water acts by its 
weight. But there is another form of mill wheels, what is called the 




Fig. 94 

undershot wheel, in which it only acts by its impact, as represented 
in Fig. 94. These are used where the height from which the water 
comes is not great enough to flow on the upper part of the wheel. 
The lower part of undershot wheels dips in the flowing water which 
strikes against their float-boards and carries them along. Such 
wheels are used in swift-flowing streams which have a scarcely per- 
ceptible fall, as, for instance, on the Rhine. In the immediate neigh- 
borhood of such a wheel, the water need not necessarily have a great 
fall if it only strikes with considerable velocity. It is the velocity of 
the water, exerting an impact against the float-boards, which acts 
in this case, and which produces the motive power. 

Windmills, which are used in the great plains of Holland and 
North Germany to supply the want of falling water, afford another 



l86 HELMHOLTZ 

instance of the action of velocity. The sails are driven by air in 
motion — by wind. Air at rest could just as little drive a windmill 
as water at rest a water wheel. The driving force depends here on 
the velocity of moving masses. 

A bullet resting in the hand is the most harmless thing in the 
world; by its gravity it can exert no great effect; but when fired and 
endowed with great velocity it drives through all obstacles with the 
most tremendous force. 

If I lay the head of a hammer gently on a nail, neither its small 
weight nor the pressure of my arm is quite sufficient to drive the 
nail into the wood; but if I swing the hammer and allow it to fall 
with great velocity, it acquires a new force, which can overcome far 
greater hindrances. 

These examples teach us that the velocity of a moving mass can 
act as motive force. In mechanics, velocity in so far as it is motive 
force, and can produce work, is called vis viva. The name is not 
well chosen; it is too apt to suggest to us the force of living beings. 
Also in this case you will see, from the instances of the hammer and 
of the bullet, that velocity is lost, as such, when it produces working 
power. In the case of the water mill, or of the windmill, a more 
careful investigation of the moving masses of water and air is 
necessary to prove that part of their velocity has been lost by the 
work which they have performed. 

The relation of velocity to working power is most simply and 
clearly seen in a simple pendulum, such as can be constructed by any 
weight which we suspend to a cord. Let M, Fig. 95, be such a weight, 
of a spherical form; A B, a horizontal line drawn through the centre 
of the sphere; P the point at which the cord is fastened. If now I 
draw the weight M on one side towards A, it moves in the arc M a, 
the end of which, a, is somewhat higher than the point A in the 
horizontal line. The weight is thereby raised to the height A a. 
Hence my arm must exert a certain force to bring the weight to a. 
Gravity resists this motion, and endeavours to bring back the weight 
to M, the lowest point which it can reach. 

Now, if after I have brought the weight to a I let it go, it obeys 
this force of gravity and returns to M, arrives there with a certain 
velocity, and no longer remains quietly hanging at M as it did 



ON THE CONSERVATION OF FORCE 1 87 

before, but swings beyond M towards b, where its motion stops as 
soon as it has traversed on the side of B an arc equal in length to 
that on the side of A, and after it has risen to a distance B b above 
the horizontal line, which is equal to the height A a, to which my 
arm had previously raised it. In b the pendulum returns, swings the 
same way back through M towards a, and so on, until its oscillations 




are gradually diminished, and ultimately annulled by the resistance 
of the air and by friction. 

You see here that the reason why the weight, when it comes from 
a to M, and does not stop there, but ascends to b, in opposition to 
the action of gravity, is only to be sought in its velocity. The velocity 
which it has acquired in moving from the height A a is capable of 
again raising it to an equal height, B b. The velocity of the moving 
mass, M, is thus capable of raising this mass; that is to say, in the 
language of mechanics, of performing work. This would also be 
the case if we had imparted such a velocity to the suspended weight 
by a blow. 

From this we learn further how to measure the working power of 
velocity — or, what is the same thing, the vis viva of the moving 



1 88 HELMHOLTZ 

mass. It is equal to the work, expressed in foot-pounds, which the 
same mass can exert after its velocity has been used to raise it, 
under the most favourable circumstances, to as great a height as 
possible.^ This does not depend on the direction of the velocity; for 
if we swing a weight attached to a thread in a circle, we can even 
change a downward motion into an upward one. 

The motion of the pendulum shows us very distinctly how the 
forms of working power hitherto considered — that of a raised weight 
and that of a moving mass — may merge into one another. In the 
points a and b, Fig. 95, the mass has no velocity; at the point M it 
has fallen as far as possible, but possesses velocity. As the weight 
goes from a torn the work of the raised weight is changed into vis 
viva; as the weight goes further from m to b the vis viva is changed 
into the work of a raised weight. Thus the work which the arm 
originally imparted to the pendulum is not lost in these oscillations 
provided we may leave out of consideration the influence of the 
resistance of the air and of friction. Neither does it increase, but it 
continually changes the form of its manifestation. 

Let us now pass to other mechanical forces, those of elastic 
bodies. Instead of the weights which drive our clocks, we find in 
timepieces and in watches, steel springs which are coiled in winding 
up the clock, and are uncoiled by the working of the clock. To 
coil up the spring we consume the force of the arm; this has to 
overcome the resisting elastic force of the spring as we wind it up, 
just as in the clock we have to overcome the force of gravity which 
the weight exerts. The coiled spring can, however, perform work; 
it gradually expends this acquired capability in driving the clock- 
work. 

If I stretch a crossbow and afterwards let it go, the stretched string 
moves the arrow; it imparts to it force in the form of velocity. To 
stretch the cord my arm must work for a few seconds; this work is 
imparted to the arrow at the moment it is shot off. Thus the cross- 
bow concentrates into an extremely short time the entire work 
which the arm had communicated in the operation of stretching; the 

^ The measure of vis viva in theoretical mechanics is half the product of the weight 
into the square of the velocity. To reduce it to the technical measure of the work 
we must divide it by the intensity of gravity; that is, by the velocity at the end of 
the first second of a freely falling body. 



ON THE CONSERVATION OF FORCE 1 89 

clock, on the contrary, spreads it over one or several days. In both 
cases no work is produced which my arm did not originally impart 
to the instrument; it is only expended more conveniently. 

The case is somewhat different if by any other natural process I 
can place an elastic body in a state of tension without having to 
exert my arm. This is possible and is most easily observed in the 
case of gases. 

If, for instance, I discharge a firearm loaded with gunpowder, 
the greater part of the mass of the powder is converted into gases 
at a very high temperature, which have a powerful tendency to 
expand, and can only be retained in the narrow space in which they 
are formed, by the exercise of the most powerful pressure. In 
expanding with enormous force they propel the bullet, and impart 
to it a great velocity, which we have already seen is a form of work. 

In this case, then, I have gained work which my arm has not 
performed. Something, however, has been lost — the gunpowder, 
that is to say, whose constituents have changed into other chemical 
compounds, from which they cannot, without further ado, be 
restored to their original condition. Here, then, a chemical change 
has taken place, under the influence of which work has been 
gained. 

Elastic forces are produced in gases by the aid of heat, on a far 
greater scale. 

Let us take, as the most simple instance, atmospheric air. In 
Fig. 96 an apparatus is represented such as Regnault used for meas- 
uring the expansive force of heated gases. If no great accuracy is 
required in the measurement, the apparatus may be arranged more 
simply. At C is a glass globe filled with dry air, which is placed 
in a metal vessel, in which it can be heated by steam. It is connected 
with the U-shaped tube, S s, which contains a liquid, and the limbs 
of which communicate with each other when the stopcock R is 
closed. If the liquid is in equilibrium in the tube S / when the globe 
is cold, it rises in the leg s, and ultimately overflows when the globe 
is heated. If, on the contrary, when the globe is heated, equilibrium 
be restored by allowing some of the liquid to flow out at R, as the 
globe cools it will be drawn up towards n. In both cases liquid is 
raised, and work thereby produced. 



190 HELMHOLTZ 

The same experiment is continuously repeated on the largest 
scale in steam engines, though, in order to keep up a continual dis- 
engagement o£ compressed gases from the boiler, the air in the globe 
in Fig. 96, which would soon reach the maximum of its expansion, 
is replaced by water, which is gradually changed into steam by the 
application of heat. But steam, so long as it remains as such, is an 




Fig. 96 

elastic gas which endeavours to expand exactly like atmospheric air. 
And instead of the column of liquid which was raised in our last 
experiment, the machine is caused to drive a solid piston which 
imparts its motion to other parts of the machine. Fig. 97 represents 
a front view of the working parts of a high-pressure engine, and 
Fig. 98 a section. The boiler in which steam is generated is not 
represented; the steam passes through the tube z z, Fig. 98, to the 
cylinder A A, in which moves a tightly fitting piston C. The parts 
between the tube z z and the cylinder A A, that is the slide valve in 
the valve-chest K K, and the two tubes d and e allow the steam to 



ON THE CONSERVATION OF FORCE 




Fig. 97 



192 HELMHOLTZ 

pass first below and then above the piston, while at the same time 
the steam has free exit from the other half of the cylinder. When 
the steam passes under the piston, it forces it upward; when the 
piston has reached the top of its course the position of the valve in 
K K changes, and the steam passes above the piston and forces it 
down again. The piston rod acts by means of the connecting rod 
P, on the crank Q of the flywheel X and sets this in motion. By 
means of the rod s, the motion of the rod regulates the opening and 
closing of the valve. But we need not here enter into those mechani- 
cal arrangements, however ingeniously they have been devised. We 
are only interested in the manner in which heat produces elastic 
vapour, and how this vapour, in its endeavour to expand, is 
compelled to move the solid parts of the machine, and furnish 
work. 

You all know how powerful and varied are the effects of which 
steam engines are capable; with them has really begun the great 
development of industry which has characterised our century before 
all others. Its most essential superiority over motive powers formerly 
known is that it is not restricted to a particular place. The store of 
coal and the small quantity of water which are the sources of its 
power can be brought everywhere, and steam engines can even be 
made movable, as is the case with steam ships and locomotives. By 
means of these machines we can develop motive power to almost an 
indefinite extent at any place on the earth's surface, in deep mines 
and even on the middle of the ocean; while water and windmills 
are bound to special parts of the surface of the land. The locomotive 
transports travellers and goods over the land in numbers and with a 
speed which must have seemed an incredible fable to our fore- 
fathers, who looked upon the mailcoach with its six passengers in 
the inside, and its ten miles an hour, as an enormous progress. 
Steam engines traverse the ocean independently of the direction of 
the wind, and, successfully resisting storms which would drive 
sailing vessels far away, reach their goal at the appointed time. The 
advantages which the concourse of numerous and variously skilled 
workmen in all branches offers in large towns where wind and water 
power are wanting, can be utilised, for steam engines find place 
everywhere, and supply the necessary crude force; thus the more 



ON THE CONSERVATION OF FORCE 



193 




Fig. 98 



194 HELMHOLTZ 

intelligent human force may be spared for better purposes; and, 
indeed, wherever the nature of the ground or the neighbourhood 
of suitable lines of communication present a favourable opportunity 
for the development of industry, the motive power is also present 
in the form of steam engines. 

We see, then, that heat can produce mechanical power; but in the 
cases which we have discussed we have seen that the quantity of 
force which can be produced by a given measure of a physical 
process is always accurately defined, and that the further capacity 
for work of the natural forces is either diminished or exhausted by 
the work which has been performed. How is it now with Heat in 
this respect? 

This question was of decisive importance in the endeavour to 
extend the law of the Conservation of Force to all natural processes. 
In the answer lay the chief difference between the older and newer 
views in these respects. Hence it is that many physicists designate 
that view of Nature corresponding to the law of the conservation of 
force with the name of Mechanical Theory of Heat. 

The older view of the nature of heat was that it is a substance, 
very fine and imponderable indeed, but indestructible, and unchange- 
able in quantity, which is an essential fundamental property of all 
matter. And, in fact, in a large number of natural processes, the 
quantity of heat which can be demonstrated by the thermometer is 
unchangeable. 

By conduction and radiation, it can indeed pass from hotter to 
colder bodies; but the quantity of heat which the former lose can 
be shown by the thermometer to have reappeared in the latter. Many 
processes, too, were known, especially in the passage of bodies from 
the solid to the liquid and gaseous states, in which heat disappeared 
— at any rate, as regards the thermometer. But when the gaseous 
body was restored to the liquid, and the liquid to the solid state, 
exactly the same quantity of heat reappeared which formerly seemed 
to have been lost. Heat was said to have become latent. On this 
view, liquid water differed from solid ice in containing a certain 
quantity of heat bound, which, just because it was bound, could not 
pass to the thermometer, and therefore was not indicated by it. 
Aqueous vapour contains a far greater quantity of heat thus bound. 



ON THE CONSERVATION OF FORCE 1 95 

But if the vapour be precipitated, and the Uquid water restored to 
the state of ice, exactly the same amount of heat is Uberated as had 
become latent in the melting of the ice and in the vaporisation of 
the water. 

Finally, heat is sometimes produced and sometimes disappears in 
chemical processes. But even here it might be assumed that the 
various chemical elements and chemical compounds contain certain 
constant quantities of latent heat, which, when they change their 
composition, are sometimes liberated and sometimes must be sup- 
plied from external sources. Accurate experiments have shown that 
the quantity of heat which is developed by a chemical process — for 
instance, in burning a pound of pure carbon into carbonic acid — is 
perfectly constant, whether the combustion is slow or rapid, whether 
it takes place all at once or by intermediate stages. This also agreed 
very well with the assumption, which was the basis of the theory of 
heat, that heat is a substance entirely unchangeable in quantity. The 
natural processes which have here been briefly mentioned, were the 
subject of extensive experimental and mathematical investigations, 
especially of the great French physicists in the last decade of the 
former, and the first decade of the present, century; and a rich and 
accurately-worked chapter of physics had been developed, in which 
everything agreed excellently with the hypothesis — that heat is a 
substance. On the other hand, the invariability in the quantity of 
heat in all these processes could at that time be explained in no other 
manner than that heat is a substance. 

But one relation of heat — namely, that to mechanical work — had 
not been accurately investigated. A French engineer, Sadi Carnot, 
son of the celebrated War Minister of the Revolution, had indeed 
endeavoured to deduce the work which heat performs, by assuming 
that the hypothetical caloric endeavoured to expand like a gas; and 
from this assumption he deduced in fact a remarkable law as to the 
capacity of heat for work, which even now, though with an essential 
alteration introduced by Clausius, is among the bases of the modern 
mechanical theory of heat, and the practical conclusions from which, 
so far as they could at that time be compared with experiments, have 
held good. 

But it was already known that whenever two bodies in motion 



196 HELMHOLTZ 

rubbed against each other, heat was developed anew, and it could 
not be said whence it came. 

The fact is universally recognised; the axle of a carriage which is 
badly greased and where the friction is great, becomes hot — so hot, 
indeed, that it may take fire; machine wheels with iron axles going 
at a great rate may become so hot that they weld to their sockets. 




A powerful degree of friction is not, indeed, necessary to disengage 
an appreciable degree of heat; thus, a lucifer match, which by rub- 
bing is so heated that the phosphoric mass ignites, teaches this fact. 
Nay, it is enough to rub the dry hands together to feel the heat 
produced by friction, and which is far greater than the heating which 
takes place when the hands lie gently on each other. Uncivilised 
people use the friction of two pieces of wood to kindle a fire. With 
this view, a sharp spindle of hard wood is made to revolve rapidly 
on a base of soft wood in the manner represented in Fig. 99. 

So long as it was only a question of the friction of solids, in which 
particles from the surface become detached and compressed, it might 



ON THE CONSERVATION OF FORCE 1 97 

be supposed that some changes in structure of the bodies rubbed 
might here Hberate latent heat, which would thus appear as heat of 
friction. 

But heat can also be produced by the friction of liquids, in which 
there could be no question of changes in structure, or of the libera- 
tion of latent heat. The first decisive experiment of this kind was 
made by Sir Humphry Davy in the commencement of the present 
century. In a cooled space he made two pieces of ice rub against 
each other, and thereby caused them to melt. The latent heat which 
the newly formed water must have here assimilated could not have 
been conducted to it by the cold ice, or have been produced by a 
change of structure; it could have come from no other cause than 
from friction, and must have been created by friction. 

Heat can also be produced by the impact of imperfectly elastic 
bodies as well as by friction. This is the case, for instance, when we 
produce fire by striking flint against steel, or when an iron bar is 
worked for some time by powerful blows of the hammer. 

If we inquire into the mechanical effects of friction and of inelastic 
impact, we find at once that these are the processes by which all 
terrestrial movements are brought to rest. A moving body whose 
motion was not retarded by any resisting force would continue to 
move to all eternity. The motions of the planets are an instance of 
this. This is apparently never the case with the motion of the 
terrestrial bodies, for they are always in contact with other bodies 
which are at rest, and rub against them. We can, indeed, very much 
diminish their friction, but never completely annul it. A wheel 
which turns about a well-worked axle, once set in motion, continues 
it for a long time; and the longer, the more truly and smoother the 
axle is made to turn, the better it is greased, and the less the pressure 
it has to support. Yet the tns viva of the motion which we have 
imparted to such a wheel when we started it, is gradually lost in con- 
sequence of friction. It disappears, and if we do not carefully con- 
sider the matter, it seems as if the vis viva which the wheel had 
possessed had been simply destroyed without any substitute. 

A bullet which is rolled on a smooth horizontal surface continues 
to roll until its velocity is destroyed by friction on the path, caused 
by the very minute impacts on its little roughnesses. 



198 HELMHOLTZ 

A pendulum which has been put in vibration can continue to 
oscillate for hours if the suspension is good, without being driven by 
a weight; but by the friction against the surrounding air, and by 
that at its place of suspension, it ultimately comes to rest. 

A stone which has fallen from a height has acquired a certain 
velocity on reaching the earth; this we know is the equivalent of a 
mechanical work; so long as this velocity continues as such, we can 
direct it upwards by means of suitable arrangements, and thus 
utilise it to raise the stone again. Ultimately the stone strikes against 
the earth and comes to rest; the impact has destroyed its velocity, and 
therewith apparently also the mechanical work which this velocity 
could have affected. 

If we review the results of all these instances, which each of you 
could easily add to from your own daily experience, we shall see 
that friction and inelastic impact are processes in which mechanical 
work is destroyed, and heat produced in its place. 

The experiments of Joule, which have been already mentioned, 
lead us a step further. He has measured in foot pounds the amount 
of work which is destroyed by the friction of solids and by the 
friction of liquids; and, on the other hand, he has determined the 
quantity of heat which is thereby produced, and has established a 
definite relation between the two. His experiments show that when 
heat is produced by the consumption of work, a definite quantity of 
work is required to produce that amount of heat which is known to 
physicists as the unit of heat; the heat, that is to say, which is neces- 
sary to raise one gramme of water through one degree centigrade. 
The quantity of work necessary for this is, according to Joule's best 
experiments, equal to the work which a gramme would perform in 
falling through a height of 425 metres. 

In order to show how closely concordant are his numbers, I will 
adduce the results of a few series of experiments which he obtained 
after introducing the latest improvements in his methods. 

I. A series of experiments in which water was heated by friction in 
a brass vessel. In the interior of this vessel a vertical axle provided 
with sixteen paddles was rotated, the eddies thus produced being 
broken by a series of projecting barriers, in which parts were cut out 
large enough for the paddles to pass through. The value of the 
equivalent was 424.9 metres. 



ON THE CONSERVATION OF FORCE 1 99 

2. Two similar experiments, in which mercury in an iron vessel 
was substituted for water in a brass one, gave 425 and 426.3 metres. 

3. Two series of experiments, in which a conical ring rubbed 
against another, both surrounded by mercury, gave 426.7 and 425.6 
metres. 

Exactly the same relations between heat and work were also found 
in the reverse process — that is, when work was produced by heat. 
In order to execute this process under physical conditions that could 
be controlled as perfectly as possible, permanent gases and not 
vapours were used, although the latter are, in practice, more con- 
venient for producing large quantities of work, as in the case of the 
steam engine. A gas which is allowed to expand with moderate 
velocity becomes cooled. Joule was the first to show the reason of 
this cooling. For the gas has, in expanding, to overcome the resist- 
ance which the pressure of the atmosphere and the slowly yielding 
side of the vessel oppose to it: or, if it cannot of itself overcome this 
resistance, it supports the arm of the observer which does it. Gas 
thus performs work, and this work is produced at the cost of its 
heat. Hence the cooling. If, on the contrary, the gas is suddenly 
allowed to issue into a perfectly exhausted space where it finds no 
resistance, it does not become cool, as Joule has shown; or if indi- 
vidual parts of it become cool, others become warm; and, after the 
temperature has become equalised, this is exactly as much as before 
the sudden expansion of the gaseous mass. 

How much heat the various gases disengage when they are com- 
pressed, and how much work is necessary for their compression; or, 
conversely, how much heat disappears when they expand under a 
pressure equal to their own counterpressure, was partly known from 
the older physical experiments, and has partly been determined by 
the recent experiments of Regnault by extremely perfect methods. 
Calculations with the best data of this kind give us the value of the 
thermal equivalent from experiments: — 

With atmospheric air 426.0 metres 

" oxygen 425.7 

" nitrogen 431.3 " 

" hydrogen . . 425.3 " 

Comparing these numbers with those which determine the equiv- 
alence of heat and mechanical work in friction, as close an agree- 



200 HELMHOLTZ 

ment is seen as can at all be expected from numbers which have 
been obtained by such varied investigations of different observers. 

Thus then: a certain quantity of heat may be changed into a 
definite quantity of work; this quantity of work can also be retrans- 
formed into heat, and, indeed, into exactly the same quantity of heat 
as that from which it originated; in a mechanical point of view, they 
are exactly equivalent. Heat is a new form in which a quantity of 
work may appear. 

These facts no longer permit us to regard heat as a substance, for 
its quantity is not unchangeable. It can be produced anew from the 
vis viva of motion destroyed; it can be destroyed, and then produces 
motion. We must rather conclude from this that heat itself is a 
motion, an internal invisible motion of the smallest elementary 
particles of bodies. If, therefore, motion seems lost in friction and 
impact, it is not actually lost, but only passes from the great visible 
masses to their smallest particles; while in steam engines the internal 
motion of the heated gaseous particles is transferred to the piston 
of the machine, accumulated in it, and combined in a resultant 
whole. 

But what is the nature of this internal motion can only be asserted 
with any degree of probability in the case of gases. Their particles 
probably cross one another in rectilinear paths in all directions, until, 
striking another particle, or against the side of the vessel, they are 
reflected in another direction. A gas would thus be analogous to a 
swarm of gnats, consisting, however, of particles infinitely small and 
infinitely more closely packed. This hypothesis, which has been 
developed by Kronig, Clausius, and Maxwell, very well accounts 
for all the phenomena of gases. 

What appeared to the earlier physicists to be the constant quantity 
of heat is nothing more than the whole motive power of the motion 
of heat, which remains constant so long as it is not transformed into 
other forms of work, or results afresh from them. 

We turn now to another kind of natural forces which can produce 
work — I mean the chemical. We have to-day already come across 
them. They are the ultimate cause of the work which gunpowder 
and the steam engine produce; for the heat which is consumed in 
the latter, for example, originates in the combustion of carbon — that 



ON THE CONSERVATION OF FORCE 201 

is to say, in a chemical process. The burning of coal is the chemical 
union of carbon with the oxygen of the air, taking place under the 
influence of the chemical affinity of the two substances. 

We may regard this force as an attractive force between the two, 
which, however, only acts through them with extraordinary power, 
if the smallest particles of the two substances are in closest proximity 
to each other. In combustion this force acts; the carbon and oxygen 
atoms strike against each other and adhere firmly, inasmuch as they 
form a new compound — carbonic acid — a gas known to all of you 
as that which ascends from all fermenting and fermented liquids — 
from beer and champagne. Now this attraction between the atoms 
of carbon and of oxygen performs work just as much as that which 
the earth in the form of gravity exerts upon a raised weight. When 
the weight falls to the ground, it produces an agitation, which is 
partly transmitted to the vicinity as sound waves, and partly remains 
as the motion of heat. The same result we must expect from chemi- 
cal action. When carbon and oxygen atoms have rushed against each 
other, the newly-formed particles of carbonic acid must be in the 
most violent molecular motion — that is, in the motion of heat. And 
this is so. A pound of carbon burned with oxygen to form carbonic 
acid, gives as much heat as is necessary to raise 80.9 pounds of water 
from the freezing to the boiling point; and just as the same amount 
of work is produced when a weight falls, whether it falls slowly or 
fast, so also the same quantity of heat is produced by the combustion 
of carbon, whether this is slow or rapid, whether it takes place all 
at once, or by successive stages. 

When the carbon is burned, we obtain in its stead, and in that of 
the oxygen, the gaseous product of combustion — carbonic acid. Im- 
mediately after combustion it is incandescent. When it has after- 
wards imparted heat to the vicinity, we have in the carbonic acid 
the entire quantity of carbon and the entire quantity of oxygen, and 
also the force of affinity quite as strong as before. But the action of 
the latter is now limited to holding the atoms of carbon and oxygen 
firmly united; they can no longer produce either heat or work any 
more than a fallen weight can do work if it has not been again raised 
by some extraneous force. When the carbon has been burnt we take 
no further trouble to retain the carbonic acid; it can do no more 



202 HELMHOLTZ 

service, we endeavour to get it out of the chimneys of our houses as 
fast as we can. 

Is it possible, then, to tear asunder the particles of carbonic acid, 
and give to them once more the capacity of work which they had 
before they were combined, just as we can restore the potentiality 
of a weight by raising it from the ground? It is indeed possible. We 
shall afterwards see how it occurs in the life of plants; it can also be 
effected by inorganic processes, though in roundabout ways, the 
explanation of which would lead us too far from our present course. 

This can, however, be easily and directly shown for another 
element, hydrogen, which can be burnt just hke carbon. Hydrogen 
with carbon is a constituent of all combustible vegetable substances, 
among others, it is also an essential constituent of the gas which is 
used for lighting our streets and rooms; in the free state it is also a 
gas, the lightest of all, and burns when ignited with a feebly lu- 
minous blue flame. In this combustion — that is, in the chemical com- 
bination of hydrogen with oxygen, a very considerable quantity of 
heat is produced; for a given weight of hydrogen, four times as much 
heat as in the combustion of the same weight of carbon. The product 
of combustion is water, which, therefore, is not of itself further 
combustible, for the hydrogen in it is completely saturated with 
oxygen. The force of affinity, therefore, of hydrogen for oxygen, 
like that of carbon for oxygen, performs work in combustion, which 
appears in the form of heat. In the water which has been formed 
during combustion, the force of affinity is exerted between the ele- 
ments as before, but its capacity for work is lost. Hence the two 
elements must be again separated, their atoms torn apart, if new 
effects are to be produced from them. 

This we can do by the aid of currents of electricity. In the appara- 
tus depicted in Fig. loo, we have two glass vessels filled with acidu- 
lated water a and a^, which are separated in the middle by a porous 
plate moistened with water. In both sides are fitted platinum wires, 
\, which are attached to platinum plates, i and /j. As soon as a 
galvanic current is transmitted through the water by the platinum 
wires, \, you see bubbles of gas ascend from the plates i and i^. These 
bubbles are the two elements of water, hydrogen on the one hand, 
and oxygen on the other. The gases emerge through the tubes g and 



ON THE CONSERVATION OF FORCE 203 

gi. If we wait until the upper part of the vessels and the tubes have 
been filled with it, we can inflame hydrogen at one side; it burns 
with a blue flame. If I bring a glimmering spill near the mouth 
of the other tube, it bursts into flame, just as happens with oxygen 
gas, in which the processes of combustion are far more intense than 




Fig. 100 

in atmospheric air, where the oxygen mixed with nitrogen is only 
one-fifth of the whole volume. 

If I hold a glass Hask filled with water over the hydrogen flame, 
the water, newly formed in combustion, condenses upon it. 

If a platinum wire be held in the almost non-luminous flame, you 
see how intensely it is ignited; in a plentiful current of a mixture 
of the gases, hydrogen and oxygen, which have been liberated in the 
above experiment, the almost infusible platinum might even be 
melted. The hydrogen which has here been liberated from the water 
by the electrical current has regained the capacity of producing large 
quantities of heat by a fresh combination with oxygen; its affinity 
for oxygen has regained for it its capacity for work. 

We here become acquainted with a new source of work, the 
electric current which decomposes water. This current is itself pro- 
duced by a galvanic battery, Fig. ioi. Each of the four vessels con- 
tains nitric acid, in which there is a hollow cylinder of very compact 
carbon. In the middle of the carbon cylinder is a cylindrical porous 
vessel of white clay, which contains dilute sulphuric acid; in this 



204 HELMHOLTZ 

dips a zinc cylinder. Each zinc cylinder is connected by a metal ring 
with the carbon cylinder of the next vessel, the last zinc cylinder, n, 
is connected with one platinum plate, and the first carbon cylinder, p, 
with the other platinum plate of the apparatus for the decomposition 
of water. 

If now the conducting circuit of this galvanic apparatus is com- 
pleted, and the decomposition of water begins, a chemical process 




Fig. ioi 



takes place simultaneously in the cells of the voltaic battery. Zinc 
takes oxygen from the surrounding water and undergoes a slow 
combustion. The product of combustion thereby produced, oxide 
of zinc, unites further with sulphuric acid, for which it has a power- 
ful affinity, and sulphate of zinc, a saline kind of substance, dissolves 
in the liquid. The oxygen, moreover, which is withdrawn from it 
is taken by the water from the nitric acid surrounding the cylinder 
of carbon, which is very rich in it, and readily gives it up. Thus, in 
the galvanic battery, zinc burns to sulphate of zinc at the cost of the 
oxygen of nitric acid. 

Thus, while one product of combustion, water, is again separated, 
a new combustion is taking place — that of zinc. While we there 
reproduce chemical affinity which is capable of work, it is here lost. 
The electrical current is, as it were, only the carrier which transfers 



ON THE CONSERVATION OF FORCE 205 

the chemical force of the zinc uniting with oxygen and acid to water 
in the decomposing cell, and uses it for overcoming the chemical 
force of hydrogen and oxygen. 




Fig. 102 

In this case, we can restore work which has been lost, but only by 
using another force, that of oxidising zinc. 

Here we have overcome chemical forces by chemical forces, 
through the instrumentality of the electrical current. But we can 



206 HELMHOLTZ 

attain the same object by mechanical forces, if we produce the elec- 
trical current by a magneto-electrical machine, Fig. 102. If we turn 
the handle, the anker R R', on which is coiled copper-wire, rotates in 
front of the poles of the horseshoe magnet, and in these coils electri- 
cal currents are produced, which can be led from the points a and b. 
If the ends of these wires are connected with the apparatus for 
decomposing water, we obtain hydrogen and oxygen, though in far 
smaller quantity than by the aid of the battery which we used 
before. But this process is interesting, for the mechanical force of 
the arm which turns the wheel produces the work which is required 
for separating the combined chemical elements. Just as the steam 
engine changes chemical into mechanical force the magneto-electrical 
machine transforms mechanical force into chemical. 

The application of electrical currents opens out a large number 
of relations between the various natural forces. We have decomposed 
water into its elements by such currents, and should be able to decom- 
pose a large number of other chemical compounds. On the other 
hand, in ordinary galvanic batteries electrical currents are produced 
by chemical forces. 

In all conductors through which electrical currents pass they pro- 
duce heat; I stretch a thin platinum wire between the ends n and p 
of the galvanic battery, Fig. ioi; it becomes ignited and melts. On 
the other hand, electrical currents are produced by heat in what are 
called thermo-electric elements. 

Iron which is brought near a spiral of copper wire, traversed by an 
electrical current, becomes magnetic, and then attracts other pieces 
of iron, or a suitably placed steel magnet. We thus obtain mechani- 
cal actions which meet with extended applications in the electrical 
telegraph, for instance. Fig. 103 represents a Morse's telegraph in 
one-third of the natural size. The essential part is a horseshoe shaped 
iron core, which stands in the copper spirals b b. Just over the top 
of this is a small steel magnet c c, which is attracted the moment an 
electrical current, arriving by the telegraph wire, traverses the spirals 
b b. The magnet c c\s rigidly fixed in the lever d d, at the other end 
of which is a style; this makes a mark on a paper band, drawn by a 
clock-work, as often and as long as c c is attracted by the magnetic 
action of the electrical current. Conversely, by reversing the mag- 



ON THE CONSERVATION OF FORCE 20/ 

netism in the iron core of the spirals b b, we should obtain in them 
an electrical current just as we have obtained such currents in the 
magneto-electrical machine, Fig. 102; in the spirals of that machine 
there is an iron core which, by being approached to the poles of the 
large horseshoe magnet, is sometimes magnetised in one and some- 
times in the other direction. 

I will not accumulate examples of such relations; in subsequent 
lectures we shall come across them. Let us review these examples 




Fig. 103 

once more, and recognise in them the law which is common to all. 

A raised weight can produce work, but in doing so it must neces- 
sarily sink from its height, and, when it has fallen as deep as it can 
fall, its gravity remains as before, but it can no longer do work. 

A stretched spring can do work, but in so doing it becomes loose. 
The velocity of a moving mass can do work, but in doing so it 
comes to rest. Heat can perform work; it is destroyed in the opera- 
tion. Chemical forces can perform work, but they exhaust them- 
selves in the effort. 

Electrical currents can perform work, but to keep them up we 
must consume either chemical or mechanical forces, or heat. 



208 HELMHOLTZ 

We may express this generally. It is a universal character of all 
}{nown natural forces that their capacity for wor\ is exhausted in 
the degree in u/hich they actually perforin wor\^. 

We have seen, further, that when a weight fell without perform- 
ing any work, it either acquired velocity or produced heat. We 
might also drive a magneto-electrical machine by a falling weight; 
it would then furnish electrical currents. 

We have seen that chemical forces, when they come into play, 
produce either heat or electrical currents or mechanical work. 

We have seen that heat may be changed into work; there are 
apparatus (thermo-electric batteries) in which electrical currents are 
produced by it. Heat can directly separate chemical compounds; 
thus, when we burn Umestone, it separates carbonic acid from lime. 

Thus, whenever the capacity for work of one natural force is 
destroyed, it is transformed into another kind of activity. Even 
within the circuit of inorganic natural forces, we can transform each 
of them into an active condition by the aid of any other natural force 
which is capable of work. The connections between the various 
natural forces which modern physics has revealed, are so extraor- 
dinarily numerous that several entirely different methods may be 
discovered for each of these problems. 

I have stated how we are accustomed to measure mechanical work, 
and how the equivalent in work of heat may be found. The equiv- 
alent in work of chemical processes is again measured by the heat 
which they produce. By similar relations, the equivalent in work 
of the other natural forces may be expressed in terms of mechanical 
work. 

If, now, a certain quantity of mechanical work is lost, there is 
obtained, as experiments made with the object of determining this 
point show, an equivalent quantity of heat, or, instead of this, of 
chemical force; and, conversely, when heat is lost, we gain an equiv- 
alent quantity of chemical or mechanical force; and, again, when 
chemical force disappears, an equivalent of heat or work; so that in 
all these interchanges between various inorganic natural forces work- 
ing force may indeed disappear in one form, but then it reappears 
in exactly equivalent quantity in some other form; it is thus neither 
increased nor diminished, but always remains in exactly the same 



ON THE CONSERVATION OF FORCE 209 

quantity. We shall subsequently see that the same law holds good 
also for processes in organic nature, so far as the facts have been 
tested. 

It follows thence that the total quantity of all the forces capable of 
work tn the whole universe remains eternal and unchanged through- 
out all their changes. All change in nature amounts to this, that 
force can change its form and locality without its quantity being 
changed. The universe possesses, once for all, a store of force which 
is not altered by any change of phenomena, can neither be increased 
nor diminished, and which maintains any change which takes place 
on it. 

You see how, starting from considerations based on the immediate 
practical interests of technical work, we have been led up to a uni- 
versal natural law, which, as far as all previous experience extends, 
rules and embraces all natural processes; which is no longer restricted 
to the practical objects of human utility, but expresses a perfectly 
general and particularly characteristic property of all natural forces, 
and which, as regards generality, is to be placed by the side of the 
laws of the unalterability of mass, and the unalterability of the 
chemical elements. 

At the same time, it also decides a great practical question which 
has been much discussed in the last two centuries, to the decision 
of which an infinity of experiments has been made and an infinity 
of apparatus constructed — that is, the question of the possibility of 
a perpetual motion. By this was understood a machine which was 
to work continuously without the aid of any external driving force. 
The solution of this problem promised enormous gains. Such a 
machine would have had all the advantages of steam without requir- 
ing the expenditure of fuel. Work is wealth. A machine which 
could produce work from nothing was as good as one which made 
gold. This problem had thus for a long time occupied the place of 
gold making, and had confused many a pondering brain. That a 
perpetual motion could not be produced by the aid of the then 
known mechanical forces could be demonstrated in the last century 
by the aid of the mathematical mechanics which had at that time 
been developed. But to show also that it is not possible even if heat, 
chemical forces, electricity, and magnetism were made to co-operate, 



210 HELMHOLTZ 

could not be done without a knowledge of our law in all its general- 
ity. The possibility of a perpetual motion was first finally negatived 
by the law of the conservation of force, and this law might also be 
expressed in the practical form that no perpetual motion is possible, 
that force cannot be produced from nothing; something must be 
consumed. 

You will only be ultimately able to estimate the importance and 
the scope of our law when you have before your eyes a series of its 
applications to individual processes in nature. 

What I have to-day mentioned as to the origin of the moving 
forces which are at our disposal, directs us to something beyond the 
narrow confines of our laboratories and our manufactories, to the 
great operations at work in the life of the earth and of the universe. 
The force of falling water can only flow down from the hills when 
rain and snow bring it to them. To furnish these, we must have 
aqueous vapour in the atmosphere, which can only be effected by the 
aid of heat, and this heat comes from the sun. The steam engine 
needs the fuel which the vegetable life yields, whether it be the still 
active life of the surrounding vegetation, or the extinct life which has 
produced the immense coal deposits in the depths of the earth. The 
forces of man and animals must be restored by nourishment; all 
nourishment comes ultimately from the vegetable kingdom, and 
leads us back to the same source. 

You see then that when we inquire into the origin of the moving 
forces which we take into our service, we are thrown back upon 
the meteorological processes in the earth's atmosphere, on the life 
of plants in general, and on the sun. 



ICE AND GLACIERS 

A LECTURE DELIVERED AT FRANKFORT-ON- 

MAIN, AND AT HEIDELBERG, IN 

FEBRUARY, 1865 

THE world of ice and of eternal snow, as unfolded to us on 
the summits of the neighbouring Alpine chain, so stern, so 
solitary, so dangerous, it may be, has yet its own peculiar 
charm. Not only does it enchain the attention of the natural phi- 
losopher, who finds in it the most wonderful disclosures as to the 
present and past history of the globe, but every summer it entices 
thousands of travellers of all conditions, who find there mental and 
bodily recreation. While some content themselves with admiring 
from afar the dazzling adornment which the pure, luminous masses 
of snowy peaks, interposed between the deeper blue of the sky and 
the succulent green of the meadows, lend to the landscape, others 
more boldly penetrate into the strange world, willingly subjecting 
themselves to the most extreme degrees of exertion and danger, if 
only they may fill themselves with the aspect of its sublimity. 

I will not attempt what has so often been attempted in vain — to 
depict in words the beauty and magnificence of nature, whose aspect 
dehghts the Alpine traveller. I may well presume that it is known 
to most of you from your own observation; or, it is to be hoped, will 
be so. But I imagine that the delight and interest in the magnificence 
of those scenes will make you the more inclined to lend a willing ear 
to the remarkable results of modern investigations on the more 
prominent phenomena of the glacial world. There we see that 
minute peculiarities of ice, the mere mention of which might at other 
times be regarded as a scientific subtlety, are the causes of the most 
important changes in glaciers; shapeless masses of rock begin to 
relate their histories to the attentive observer, histories which often 



212 HELMHOLTZ 

stretch far beyond the past of the human race into the obscurity of 
the primeval world; a peaceful, uniform, and beneficent sway of 
enormous natural forces, where at first sight only desert wastes are 
seen, either extended indefinitely in cheerless, desolate solitudes, or 
full of wild, threatening confusion — an arena of destructive forces. 
And thus I think I may promise that the study of the connection 
of those phenomena of which I can now only give you a very short 
outline, will not only afford you some prosaic instruction, but will 
make your pleasure in the magnificent scenes of the high mountains 
more vivid, your interest deeper, and your admiration more exalted. 

Let me first of all recall to your remembrance the chief features 
of the external appearance of the snow fields and of the glaciers; and 
let me mention the accurate measurements which have contributed 
to supplement observation, before I pass to discuss the casual con- 
nection of those processes. 

The higher we ascend the mountains the colder it becomes. Our 
atmosphere is like a warm covering spread over the earth; it is well- 
nigh entirely transparent for the luminous darting rays of the sun, 
and allows them to pass almost without appreciable change. But it 
is not equally penetrable by obscure heat rays, which, proceeding 
from heated terrestrial bodies, struggle to diffuse themselves into 
space. These are absorbed by atmospheric air, especially when it is 
moist; the mass of air is itself heated thereby, and only radiates 
slowly into space the heat which has been gained. The expenditure 
of heat is thus retarded as compared with the supply, and a certain 
store of heat is retained along the whole surface of the earth. But 
on high mountains the protective coating of the atmosphere is far 
thinner — the radiated heat of the ground can escape thence more 
freely into space; there, accordingly, the accumulated store of heat 
and the temperature are far smaller than at lower levels. 

To this must be added another property of air which acts in the 
same direction. In a mass of air which expands, part of its store of 
heat disappears; it becomes cooler, if it cannot acquire fresh heat 
from without. Conversely, by renewed compression of the air, the 
same quantity of heat is reproduced which had disappeared during 
expansion. Thus if, for instance, south winds drive the warm air of 
the Mediterranean towards the north, and compel it to ascend along 



ICE AND GLACIERS 213 

the great mountain wall of the Alps, where the air, in consequence 
of the diminished pressure, expands by about half its volume, it 
thereby becomes very greatly cooled — for a mean height of 11,000 
feet, by from 18° to 30° C, according as it is moist or dry — and it 
thereby deposits the greater part of its moisture as rain or snow. If 
the same wind, passing over to the north side of the mountains as 
Fohn-wind, reaches the valleys and plains, it again becomes con- 
densed, and is again heated. Thus the same current of air which is 
warm in the plains, both on this side of the chain and on the other, 
is bitterly cold on the heights, and can there deposit snow, while in 
the plain we find it insupportably hot. 

The lower temperature at greater heights, which is due to both 
these causes, is, as we know, very marked on the lower mountain 
chains of our neighbourhood. In central Europe it amounts to about 
1° C. for an ascent of 480 feet; in winter it is less — 1° for about 720 
feet of ascent. In the Alps the differences of temperature at great 
heights are accordingly far more considerable, so that upon the 
higher parts of their peaks and slopes the snow which has fallen in 
winter no longer melts in summer. This line, above which snow 
covers the ground throughout the entire year, is well known as the 
snow line; on the northern side of the Alps it is about 8,000 feet 
high, on the southern side about 8,800 feet. Above the snow line it 
may on sunny days be very warm ; the unrestrained radiation of the 
sun, increased by the light reflected from the snow, often becomes 
utterly unbearable, so that the tourist of sedentary habits, apart 
from the dazzling of his eyes, which he must protect by dark spec- 
tacles or by a veil, usually gets severely sunburnt in the face and 
hands, the result of which is an inflammatory swelling of the skin 
and great blisters on the surface. More pleasant testimonies to the 
power of the sunshine are the vivid colours and the powerful odour 
of the small Alpine flowers which bloom in the sheltered rocky clefts 
among the snow fields. Notwithstanding the powerful radiation of 
the sun the temperature of the air above the snow fields only rises 
to 5°, or at most 8°; this, however, is sufficient to melt a tolerable 
amount of the superficial layers of snow. But the warm hours and 
days are too short to overpower the great masses of snow which have 
fallen during colder times. Hence the height of the snow line does 



214 HELMHOLTZ 

not depend merely on the temperature of the mountain slope, but 
also essentially on the amount of the yearly snowfall. It is lower, 
for instance, on the moist and warm south slope of the Himalayas 
than on the far colder but also far drier north slope of the same 
mountain. Corresponding to the moist climate of western Europe, 
the snowfall upon the Alps is very great, and hence the number and 
extent of their glaciers are comparatively considerable, so that few 
mountains of the earth can be compared with them in this respect. 
Such a development of the glacial world is, as far as we know, met 
with only on the Himalayas, favoured by the greater height; in 
Greenland and in Northern Norway, owing to the colder climate; 
in a few islands in Iceland; and in New Zealand, from the more 
abundant moisture. 

Places above the snow line are thus characterised by the fact that 
the snow which in the course of the year falls on its surface does not 
quite melt away in summer, but remains to some extent. This snow, 
which one summer has left, is protected from the further action of 
the sun's heat by the fresh quantities that fall upon it during the 
next autumn, winter, and spring. Of this new snow also next sum- 
mer leaves some remains, and thus year by year fresh layers of snow 
are accumulated one above the other. In those places where such an 
accumulation of snow ends in a steep precipice, and its inner struc- 
ture is thereby exposed, the regularly stratified yearly layers are 
easily recognised. 

But it is clear that this accumulation of layer upon layer cannot go 
on indefinitely, for otherwise the height of the snow peak would 
continually increase year by year. But the more the snow is accumu- 
lated the steeper are the slopes, and the greater the weight which 
presses upon the lower and older layers and tries to displace them. 
Ultimately a state must be reached in which the snow slopes are too 
steep to allow fresh snow to rest upon them, and in which the 
burden which presses the lower layers downwards is so great that 
these can no longer retain their position on the sides of the moun- 
tain. Thus, part of the snow which had originally fallen on the 
higher regions of the mountain above the snow line, and had there 
been protected from melting, is compelled to leave its original posi- 
tion and seek a new one, which it of course finds only below the 



ICE AND GLACIERS 21 5 

snow line on the lower slopes of the mountain, and especially in the 
valleys, where, however, being exposed to the influence of a warmer 
air, it ultimately melts and flows away as water. The descent of 
masses of snow from their original positions sometimes happens 
suddenly in avalanches, but it is usually very gradual in the form of 
glaciers. 

Thus we must discriminate between two distinct parts of the ice 
fields; that is, first, the snow which originally fell — called firn in 
Switzerland — above the snow line, covering the slopes of the peaks 
as far as it can hang on to them, and filling up the upper wdde 
kettle-shaped ends of the valleys forming widely extending fields of 
snow or firnmeere. Secondly, the glaciers, called in the Tyrol firner, 
which as prolongations of the snow fields often extend to a distance 
of from 4,000 to 5,000 feet below the snow line, and in which the 
loose snow of the snow fields is again found changed into trans- 
parent solid ice. Hence the name glacier, which is derived from the 
Latin, glacies; French, glace, glacier. 

The outward appearance of glaciers is very characteristically 
described by comparing them, with Goethe, to currents of ice. They 
generally stretch from the snow fields along the depth of the valleys, 
filling them throughout their entire breadth, and often to a con- 
siderable height. They thus follow all the curvatures, windings, con- 
tractions, and enlargements of the valley. Two glaciers frequently 
meet the valleys of which unite. The two glacial currents then join 
in one common principal current, filling up the valley common to 
them both. In some places these ice currents present a tolerably level 
and coherent surface, but they are usually traversed by crevasses, and 
both over the surface and through the crevasses countless small and 
large water rills ripple, which carry off the water formed by the 
melting of the ice. United, and forming a stream, they burst, through 
a vaulted and clear blue gateway of ice, out at the lower end of the 
larger glacier. 

On the surface of the ice there is a large quantity of blocks of 
stone, and of rocky debris, which at the lower end of the glacier are 
heaped up and form immense walls; these are called the lateral and 
terminal moraine of the glacier. Other heaps of rock, the central 
moraine, stretch along the surface of the glacier in the direction of its 



2l6 HELMHOLTZ 

length, forming long regular dark lines. These always start from 
the places where two glacier streams coincide and unite. The central 
moraines are in such places to be regarded as the continuations of 
the united lateral moraines of the two glaciers. 

The formation of the central moraine is well represented in the 
view below given of the Unteraar Glacier (Fig. 104). In the back- 
ground are seen the two glacier currents emerging from different 




Fig. 104 

valleys; on the right from the Shreckhorn, and on the left from the 
Finsteraarhorn. From the place where they unite the rocky wall 
occupying the middle of the picture descends, constituting the cen- 
tral moraine. On the left are seen individual large masses of rock 
resting on pillars of ice, which are known as glacier tables. 

To exemplify these circumstances still further, I lay before you in 
Fig. 105 a map of the Mer de Glace of Chamouni, copied from that 
of Forbes. 

The Mer de Glace in size is well known as the largest glacier in 
Switzerland, although in length it is exceeded by the Aletsch Glacier. 



ICE AND GLACIERS 217 

It is formed from the snow fields that cover the heights directly 
north of Mont Blanc, several of which, as the Grande Jorasse, the 
Aiguille Verte (a, Figs. 105 and 106), the Aiguille du Geant (b), 
Aiguille du Midi (c), and the Aiguille du Dru (d), are only 2,000 
to 3,000 feet below that king of the European mountains. The snow 
fields which lie on the slopes and in the basins between these moun- 
tains collect in three principal currents, the Glacier du Geant, Glacier 
de Lechaud, and Glacier du Talefre, which, ultimately united as 
represented in the map, form the Mer de Glace; this stretches as an 
ice current 2,600 to 3,000 feet in breadth down into the valley of 
Chamouni, where a powerful stream, the Arveyron, bursts from its 
lower end at k, and plunges into the Arve. The lowest precipice of 
the Mer de Glace, which is visible from the valley of Chamouni, and 
forms a large cascade of ice, is commonly called Glacier des Bois, 
from a small village which lies below. 

Most of the visitors at Chamouni only set foot on the lowest part 
of the Mer de Glace from the inn at the Montanvert, and when they 
are free from giddiness cross the glacier at this place to the little 
house on the opposite side, the Chapeau (n). Although, as the map 
shows, only a comparatively very small portion of the glacier is thus 
seen and crossed, this way shows sufficiently the magnificent scenes, 
and also the difficulties of a glacier excursion. Bolder wanderers 
march upwards along the glacier to the Jardin, a rocky cliff clothed 
with some vegetation, which divides the glacial current of the Glacier 
du Talefre into two branches; and bolder still they ascend yet higher, 
to the Col du Geant (11,000 feet above the sea), and down the Italian 
side to the valley of Aosta. 

The surface of the Mer de Glace shows four of the rocky walls 
which we have designated as medial moraines. The first, nearest the 
side of the glacier, is formed where the two arms of the Glacier du 
Talefre unite at the lower end of the Jardin; the second proceeds 
from the union of the glacier in question with the Glacier de 
Lechaud; the third, from the union of the last with the Glacier du 
Geant; and the fourth, finally, from the top of the rock ledge which 
stretches from the Aiguille du Geant towards the cascade (g) of 
the Glacier du Geant. 

To give you an idea of the slope and the fall of the glacier, I have 



2l8 HELMHOLTZ 

given in Fig. io6 a longitudinal section of it according to the levels 
and measurements taken by Forbes, with the view of the right bank 
of the glacier. The letters stand for the same objects as in Fig. 105; 
p is the Aiguille de Lechaud, q the Aiguille Noire, r the Mont Tacul, 
f is the Col du Geant, the lowest point in the high wall of rock that 
surrounds the upper end of the snow fields which feed the Mer de 
Glace. The base line corresponds to a length of a little more than 



^^n</f 



'eJorn 



#! 



^"l^ii.* 



#«'^^S*"^fM 




Fig. 105 

nine miles : on the right the heights above the sea are given in feet. 
The drawing shows very distinctly how small in most places is the 
fall of the glacier. Only an approximate estimate could be made of 
the depth, for hitherto nothing certain has been made out in refer- 
ence to it. But that it is very deep is obvious from the following 
individual and accidental observations. 

At the end of a vertical rock wall of the Tacul, the edge of the 
Glacier du Geant is pushed forth, forming an ice wall 140 feet in 
height. This would give the depth of one of the upper arms of the 
glacier at the edge. In the middle and after the union of the three 



ICE AND GLACIERS 219 

glaciers the depth must be far greater. Somewhat below the junction 
Tyndall and Hirst sounded a moulin, that is, a cavity through which 
the surface glacier waters escape, to a depth of 160 feet; the guides 
alleged that they had sounded a similar aperture to a depth of 350 
feet, and had found no bottom. From the usually deep trough- 
shaped or gorge-like form of the bottom of the valleys, which is con- 
structed solely of rock walls, it seems improbable that for a breadth 
of 3,000 feet the mean depth should only be 350 feet; moreover, from 
the manner in which ice moves, there must necessarily be a very thick 
coherent mass beneath the crevassed part. 

To render these magnitudes more intelligible by reference to more 
familiar objects, imagine the valley of Heidelberg filled with ice up 

a , 

4 A. P A^ 

Fig. 106 

to the Molkencur, or higher, so that the whole town, with all its 
steeples and the castle, is buried deeply beneath it; if, further, you 
imagine this mass of ice, gradually extending in height, continued 
from the mouth of the valley up to Neckargemiind, that would about 
correspond to the lower united ice current of the Mer de Glace. Or, 
instead of the Rhine and the Nahe at Bingen, suppose two ice 
currents united which fill the Rhine valley to its upper border as 
far as we can see from the river, and then the united currents stretch- 
ing downwards to beyond Asmannshausen and Burg Rheinstein; 
such a current would also about correspond to the size of the Mer 
de Glace. 

Fig. 107, which is a view of the magnificent Gorner Glacier seen 
from below, also gives an idea of the size of the masses of ice of the 
larger glaciers. 

The surface of most glaciers is dirty, from the numerous pebbles 
and sand which lie upon it, and which are heaped together the more 
the ice under them and among them melts away. The ice of the 



220 



HELMHOLTZ 



surface has been partially destroyed and rendered crumbly. In the 
depths of the crevasses ice is seen of a purity and clearness with 
which nothing that we are acquainted with on the plains can be 
compared. From its purity it shows a splendid blue, like that of 
the sky, only with a greenish hue. Crevasses in which pure ice is 
visible in the interior occur of all sizes; in the beginning they form 
slight cracks in which a knife can scarcely be inserted; becoming 
gradually enlarged to chasms, hundreds or even thousands, of feet 




Fig. 107 

in length, and twenty, fifty, and as much as a hundred feet in breadth, 
while some of them are immeasurably deep. Their vertical dark 
blue walls of crystal ice, glistening with moisture from the trickling 
water, form one of the most splendid spectacles which nature can 
present to us; but, at the same time, a spectacle strongly impregnated 
with the excitement of danger, and only enjoyable by the traveller 
who feels perfectly free from the slightest tendency to giddiness. The 
tourist must know how, with the aid of well-nailed shoes and a 
pointed Alpenstock, to stand even on slippery ice, and at the edge 
of a vertical precipice the foot of which is lost in the darkness of 
night, and at an unknown depth. Such crevasses cannot always be 
evaded in crossing the glacier; at the lower part of the Mer de Glace, 
for instance, where it is usually crossed by travellers, we are com- 
pelled to travel along some extent of precipitous banks of ice which 



ICE AND GLACIERS 221 

are occasionally only four to six feet in breadth, and on each side 
of which is such a blue abyss. Many a traveller, who has crept along 
the steep rocky slopes without fear, there feels his heart sink, and 
cannot turn his eyes from the yawning chasm, for he must first 
carefully select every step for his feet. And yet these blue chasms, 
which lie open and exposed in the daylight, are by no means the 
worst dangers of the glacier; though, indeed, we are so organised 
that a danger which we perceive, and which therefore we can safely 
avoid, frightens us far more than one which we know to exist, but 
which is veiled from our eyes. So also it is with glacier chasms. In 
the lower part of the glacier they yawn before us, threatening death 
and destruction, and lead us, timidly collecting all our presence of 
mind, to shrink from them; thus accidents seldom occur. On the 
upper part of the glacier, on the contrary, the surface is covered with 
snow; this, when it falls thickly, soon arches over the narrower crev- 
asses of a breadth of from four to eight feet, and forms bridges which 
quite conceal the crevasse, so that the traveller only sees a beautiful 
plane snow surface before him. If the snow bridges are thick enough, 
they will support a man; but they are not always so, and these are 
the places where men, and even chamois, are so often lost. These 
dangers may readily be guarded against if two or three men are 
roped together at intervals of ten or twelve feet. If then one of them 
falls into a crevasse, the two others can hold him, and draw him out 
again. 

In some places the crevasses may be entered, especially at the lower 
end of a glacier. In the well-known glaciers of Grindelwald, Rosen- 
laui, and other places, this is facilitated by cutting steps and arrang- 
ing wooden planks. Then any one who does not fear the perpetually 
trickling water may explore these crevasses, and admire the wonder- 
fully transparent and pure crystal walls of these caverns. The 
beautiful blue colour which they exhibit is the natural colour of 
perfectly pure water; liquid water as well as ice is blue, though to an 
extremely small extent, so that the colour is only visible in layers of 
from ten to twelve feet in thickness. The water of the Lake of 
Geneva and of the Lago di Garda exhibits the same splendid colour 
as ice. 

The glaciers are not everywhere crevassed; in places where the ice 



222 



HELMHOLTZ 



meets with an obstacle, and in the middle of great glacier currents 
the motion of which is uniform, the surface is perfectly coherent. 

Fig. io8 represents one of the more level parts of the Mer de Glace 
at the Montanvert, the little house of which is seen in the back- 




FiG. io8 

ground. The Gries Glacier, where it forms the height of the pass 
from the Upper Rhone valley to the Tosa valley, may even be crossed 
on horseback. We find the greatest disturbance of the surface of the 
glacier in those places where it passes from a slightly inclined part 
of its bed to one where the slope is steeper. The ice is there torn in 
all directions into a quantity of detached blocks, which by melting 



ICE AND GLACIERS 223 

are usually changed into wonderfully shaped sharp ridges and pyra- 
mids, and from time to time fall into the interjacent crevasses with a 
loud rumbling noise. Seen from a distance such a place appears like 
a wild frozen waterfall, and is therefore called a cascade; such a 
cascade is seen in the Glacier du Talefre at I, another is seen in the 




109 



Glacier du Geant at g, Fig, iio, while a third forms the lower end 
of the Mer de Glace. The latter, already mentioned as the Glacier 
des Bois, which rises directly from the trough of the valley at 
Chamouni to a height of 1,700 feet, the height of the Konigstuhl at 
Heidelberg, affords at all times a chief object of admiration to the 
Chamouni tourist. Fig 109 represents a view of its fantastically rent 
blocks of ice. 



224 HELMHOLTZ 

We have hitherto compared the glacier with a current as regards 
its outer form and appearance. This similarity, however, is not 
merely an external one: the ice of the glacier does, indeed, move 
forwards like the water of a stream, only more slowly. That this 
must be the case follows from the considerations by which I have 
endeavoured to explain the origin of a glacier. For as the ice is 
being constantly diminished at the lower end by melting, it would 
entirely disappear if fresh ice did not continually press forward from 
above, which, again, is made up by the snowfalls on the mountain 
tops. 

But by careful ocular observation we may convince ourselves that 
the glacier does actually move. For the inhabitants of the valleys, 
who have the glaciers constantly before their eyes, often cross them, 
and in so doing make use of the larger blocks of stone as sign posts — 
detect this motion by the fact that their guide posts gradually descend 
in the course of each year. And as the yearly displacement of the 
lower half of the Mer de Glace at Chamouni amounts to no less than 
from 400 to 600 feet, you can readily conceive that such displacements 
must ultimately be observed, notwithstanding the slow rate at which 
they take place, and in spite of the chaotic confusion of crevasses and 
rocks which the glacier exhibits. 

Besides rocks and stones, other objects which have accidentally 
alighted upon the glacier are dragged along. In 1788 the celebrated 
Genevese Saussure, together with his son and a company of guides 
and porters, spent sixteen days on the Col du Geant. On descending 
the rocks at the side of the cascade of the Glacier du Geant, they left 
behind them a wooden ladder. This was at the foot of the Aiguille 
Noire, where the fourth band of the Mer de Glace begins; this line 
thus marks at the same time the direction in which ice travels from 
this point. In the year 1832, that is, forty-four years after, fragments 
of this ladder were found by Forbes and other travellers not far 
below the junction of the three glaciers of the Mer de Glace, in the 
same line (at s. Fig. no), from which it results that these parts of 
the glacier must on the average have each year descended 375 feet. 

In the year 1827 Hugi had built a hut on the central moraine of 
the Unteraar Glacier for the purpose of making observations; the 
exact position of this hut was determined by himself and afterwards 



ICE AND GLACIERS 225 

by Agassiz, and they found that each year it had moved downwards. 
Fourteen years later, in the year 1841, it was 4,884 feet lower, so that 
every year it had on the average moved through 349 feet. Agassiz 
afterwards found that his own hut, which he had erected on the 
same glacier, had moved to a somewhat smaller extent. For these 
observations a long time was necessary. But if the motion of the 
glacier be observed by means of accurate measuring instruments, 




Fig. no 

such as theodolites, it is not necessary to wait for years to observe 
that ice moves — a single day is sufficient. 

Such observations have in recent times been made by several 
observers, especially by Forbes and by Tyndall. They show that in 
summer the middle of the Mer de Glace moves through twenty 
inches a day, while towards the lower terminal cascade the motion 
amounts to as much as thirty-five inches in a day. In winter the 
velocity is only about half as great. At the edges and in the lower 
layers of the glacier, as in a flow of water, it is considerably smaller 
than in the centre of the surface. 



226 HELMHOLTZ 

The upper sources of the Mer de Glace also have a slower motion, 
the Glacier du Geant thirteen inches a day, and the Glacier du 
Lechaud nine inches and a half. In different glaciers the velocity is 
in general very various, according to the size, the inclination, the 
amount of snowfall, and other circumstances. 

Such an enormous mass of ice thus gradually and gently moves on, 
imperceptibly to the casual observer, about an inch an hour — the ice 
of the Col du Geant will take 120 years before it reaches the lower 
end of the Mer de Glace — but it moves forward with uncontrollable 
force, before which any obstacles that man could oppose to it yield 
like straws, and the traces of which are distinctly seen even on the 
granite walls of the valley. If, after a series of wet seasons, and an 
abundant fall of snow on the heights, the base of a glacier advances, 
not merely does it crush dwelling houses, and break the trunks of 
powerful trees, but the glacier pushes before it the boulder walls 
which form its terminal moraine without seeming to experience any 
resistance. A truly magnificent spectacle is this motion, so gentle 
and so continuous, and yet so powerful and so irresistible. 

I will mention here that from the way in which the glacier moves 
we can easily infer in what places and in what directions crevasses 
will be formed. For as all layers of the glacier do not advance with 
equal velocity, some points remain behind others; for instance, the 
edges as compared with the middle. Thus if we observe the distance 
from a given point at the edge to a given point of the middle, both 
of which were originally in the same line, but the latter of which 
afterwards descended more rapidly, we shall find that this distance 
continually increases; and since the ice cannot expand to an extent 
corresponding to the increasing distance, it breaks up and forms 
crevasses, as seen along the edge of the glacier in Fig. hi, which 
represents the Gorner Glacier at Zermatt. It would lead me too far 
if I were here to attempt to give a detailed explanation of the forma- 
tion of the more regular system of crevasses, as they occur in certain 
parts of all glaciers; it may be sufficient to mention that the con- 
clusions deducible from the considerations above stated are fully 
borne out by observation. 

I will only draw attention to one point — what extremely small 
displacements are su£Scient to cause ice to form hundreds of crevasses. 



ICE AND GLACIERS 227 

The section of the Mer de Glace (Fig. 112, at g, c, h) shows places 
where a scarcely perceptible change in the inclination of the surface 
of the ice occurs of from two to four degrees. This is sufficient to 
produce a system of cross crevasses on the surface. Tyndall more 
especially has urged and confirmed by observation and measure- 
ments, that the mass of ice of the glacier does not give way in the 




Fig. Ill 

smallest degree to extension, but when subjected to a pull is invari- 
ably torn asunder. 

The distribution of the boulders, too, on the surface of the glacier 
is readily explained when we take their motion into account. These 
boulders are fragments of the mountains between which the glacier 
flows. Detached partly by the weathering of the stone, and partly by 
the freezing of water in its crevices, they fall, and for the most part 
on the edge of the mass of ice. There they either remain lying on 
the surface, or if they have originally burrowed in the snow, they 
ultimately reappear in consequence of the melting of the super- 
ficial layers of ice and snow, and they accumulate especially at the 
lower end of the glacier, where more of the ice between them has 



228 HELMHOLTZ 

been melted. The blocks which are gradually borne down to the 
lower end of the glacier are sometimes quite colossal in size. Solid 
rocky masses of this kind are met with in the lateral and terminal 
moraines, which are as large as a two-storied house. 

The masses of stone move in lines which are always nearly parallel 
to each other and to the longitudinal direction of the glacier. Those, 
therefore, that are already in the middle remain in the middle, and 
those that lie on the edge remain at the edge. These latter are the 
more numerous, for during the entire course of the glacier fresh 
boulders are constantly falling on the edge, but cannot fall on the 
middle. Thus are formed on the edge of the mass of ice the lateral 
moraines, the boulders of which partly move along with the ice, 
partly glide over its surface, and partly rest on the solid rocky base 




oo««r 



000' 



Fig. 112 

near the ice. But when two glacier streams unite, their coinciding 
lateral moraines come to lie upon the centre of the united ice-stream, 
and then move forward as central moraines parallel to each other 
and to the banks of the stream, and they show, as far as the lower 
end, the boundary line of the ice which originally belonged to one or 
the other of the arms of the glacier. They are very remarkable as 
displaying in what regular parallel bands the adjacent parts of the 
ice-stream glide downwards. A glance at the map of the Mer de 
Glace, and its four central moraines, exhibits this very distinctly. 

On the Glacier du Geant and its continuation in the Mer de Glace, 
the stones on the surface of the ice delineate, in alternately greyer 
and whiter bands, a kind of yearly rings which were first observed 
by Forbes. For since in the cascade at g, Fig. 112, more ice slides 
down in summer than in winter, the surface of the ice below the 
cascade forms a series of terraces as seen in the drawing, and as those 
slopes of the terraces which have a northern aspect melt less than 



ICE AND GLACIERS 229 

their upper plane surfaces, the former exhibit purer ice than the lat- 
ter. This, according to Tyndall, is the probable origin of these dirt 
bands. At first they run pretty much across the glacier, but as after- 
wards their centre moves somewhat more rapidly than the ends, they 
acquire farther down a curved shape, as represented in the map, 
Fig. 1 10. By their curvature they thus show to the observer with 
what varying velocity ice advances in the different parts of its course. 

A very peculiar part is played by certain stones which are imbedded 
in the lower surface of the mass of ice, and which have partly fallen 
there through crevasses, and may partly have been detached from 
the bottom of the valley. For these stones are gradually pushed with 
the ice along the base of the valley, and at the same time are pressed 
against this base by the enormous weight of the superincumbent ice. 
Both the stones imbedded in the ice as well as the rocky base are 
equally hard, but by their friction against each other they are ground 
to powder with a power compared to which any human exertion of 
force is infinitely small. The product of this friction is an extremely 
fine powder, which, swept away by water, appears lower down in 
the glacier brook, imparting to it a whitish or yellowish muddy 
appearance. 

The rocks of the trough of the valley, on the contrary, on which 
the glacier exerts year by year its grinding power, are polished as if 
in an enormous polishing machine. They remain as rounded, 
smoothly polished masses, in which are occasional scratches pro- 
duced by individual harder stones. Thus we see them appear at the 
edge of existing glaciers, when after a series of dry and hot seasons 
the glaciers have somewhat receded. But we find such polished rocks 
as remains of gigantic ancient glaciers to a far greater extent in the 
lower parts of many Alpine valleys. In the valley of the Aar more 
especially, as far down as Meyringen, the rock-walls polished to a 
considerable height are very characteristic. There also we find the 
celebrated polished plates, over which the way passes, and which are 
so smooth that furrows have had to be hewn into them and rails 
erected to enable men and animals to traverse them in safety. 

The former enormous extent of glaciers is recognised by ancient 
moraine-dykes and by transported blocks of stone, as well as by these 
polished rocks. The blocks of stone which have been carried away 



230 HELMHOLTZ 

by the glacier are distinguished from those which water has rolled 
down, by their enormous magnitude, by the perfect retention of all 
their edges which are not at all rounded off, and finally by their 
being deposited on the glacier in exactly the same order in which the 
rocks of which they formed part stand in the mountain ridge; while 
the stones which currents of water carry along are completely mixed 
together. 

From these indications, geologists have been able to prove that 
the glaciers of Chamouni, of Monte Rosa, of the St. Gotthard, and 
the Bernese Alps, formerly penetrated through the valley of the 
Arve, the Rhone, the Aar, and the Rhine to the more level part of 
Switzerland and the Jura, where they have deposited their boulders 
at a height of more than a thousand feet above the present level of 
the lake of Neufchatel. Similar traces of ancient glaciers are found 
upon the mountains of the British Islands, and upon the Scandi- 
navian Peninsula. 

The drift ice too of the Arctic Sea is glacier ice; it is pushed down 
into the sea by the glaciers of Greenland, becomes detached from the 
rest of the glacier, and floats away. In Switzerland we find a similar 
formation of drift ice, though on a far smaller scale, in the little 
Marjelen See, into which part of the ice of the great Aletsch Glacier 
pushes down. Blocks of stone which lie in drift ice may make long 
voyages over the sea. The vast number of blocks of granite which 
are scattered on the North German plains, and whose granite belongs 
to the Scandinavian mountains, has been transported by drift ice at 
the time when the European glaciers had such an enormous extent. 

I must unfortunately content myself with these few references to 
the ancient history of glaciers, and revert now to the processes at 
present at work in them. 

From the facts which I have brought before you it results that the 
ice of a glacier flows slowly like the current of a very viscous sub- 
stance, such for instance as honey, tar, or thick magma of clay. The 
mass of ice does not merely flow along the ground like a solid which 
glides over a precipice, but it bends and twists in itself; and although 
even while doing this it moves along the base of the valley, yet the 
parts which are in contact with the bottom and the sides of the valley 
are perceptibly retarded by the powerful friction; the middle of the 



ICE AND GLACIERS 23 1 

surface of the glacier, which is most distant both from the bottom 
and the sides, moving most rapidly. Rendu, a Savoyard priest, and 
the celebrated natural philosopher Forbes, were the first to suggest 
the similarity of a glacier with a current of a viscous substance. 

Now you will perhaps inquire with astonishment how it is possible 
that ice, which is the most brittle and fragile of substances, can flow 
in the glacier like a viscous mass; and you may perhaps be disposed 
to regard this as one of the wildest and most improbable statements 
that have ever been made by philosophers. I will at once admit that 
philosophers themselves were not a little perplexed by these results 
of their investigations. But the facts were there, and could not be 
got rid of. How this mode of motion originated was for a long time 
quite enigmatical, the more so since the numerous crevasses in gla- 
ciers were a sufficient indication of the well-known brittleness of 
ice; and as Tyndall correctly remarked, this constituted an essential 
difference between a stream of ice and the flow of lava, of tar, of 
honey, or of a current of mud. 

The solution of this strange problem was found, as is so often the 
case in the natural sciences, in apparently recondite investigations 
into the nature of heat, which form one of the most important con- 
quests of modern physics, and which constitute what is known as 
the mechanical theory of heat. Among a great number of deductions 
as to the relations of the diverse natural forces to each other, the 
principles of the mechanical theory of heat lead to certain con- 
clusions as to the dependence of the freezing point of water on the 
pressure to which ice and water are exposed. 

Every one knows that we determine that one fixed point of our 
thermometer scale which we call the freezing point or zero by plac- 
ing the thermometer in a mixture of pure water and ice. Water, at 
any rate when in contact with ice, cannot be cooled below zero with- 
out itself being converted into ice; ice cannot be heated above 
the freezing point without melting. Ice and water can exist in 
each other's presence at only one temperature, the temperature of 
zero. 

Now, if we attempt to heat such a mixture by a flame beneath it, 
the ice melts, but the temperature of the mixture is never raised above 
that of 0° so long as some of the ice remains unmelted. The heat 



232 HELMHOLTZ 

imparted changes ice at zero into water at zero, but the thermom- 
eter indicates no increase of temperature. Hence physicists say that 
heat has become latent, and that water contains a certain quantity of 
latent heat beyond that of ice at the same temperature. 

On the other hand, when we withdraw more heat from the mix- 
ture of ice and water, the water gradually freezes; but as long as 
there is still liquid water, the temperature remains at zero. Water 
at 0° has given up its latent heat, and has become changed into 
ice at 0°. 

Now a glacier is a mass of ice which is everywhere interpenetrated 
by water, and its internal temperature is therefore everywhere that 
of the freezing point. The deeper layers, even of the fields of neve, 
appear on the heights which occur in our Alpine chain to have every- 
where the same temperature. For, though the freshly fallen snow of 
these heights is, for the most part, at a lower temperature than that 
of 0°, the first hours of warm sunshine melt its surface and form 
water, which trickles into the deeper colder layers, and there freezes, 
until it has throughout been brought to the temperature of the 
freezing point. This temperature then remains unchanged. For, 
though by the sun's rays the surface of the ice may be melted, it 
cannot be raised above zero, and the cold of winter penetrates as 
little into the badly conducting masses of snow and ice as it does 
into our cellars. Thus the interior of the masses of neve, as well as 
of the glacier, remains permanently at the melting point. 

But the temperature at which water freezes may be altered by 
strong pressure. This was first deduced from the mechanical theory 
of heat by James Thomson of Belfast, and almost simultaneously by 
Clausius of Ziirich; and, indeed, the amount of this change may be 
correctly predicted from the same reasoning. For each increase of a 
pressure of one atmosphere the freezing point is lowered by the 
i-ii5th part of a degree Centigrade. The brother of the former. Sir 
W. Thomson, the celebrated Glasgow physicist, made an experi- 
mental confirmation of this theoretical deduction by compressing in 
a suitable vessel a mixture of ice and snow. This mixture became 
colder and colder as the pressure was increased, and to the extent 
required by the mechanical theory. 

Now, if a mixture of ice and water becomes colder when it is 



ICE AND GLACIERS 233 

subjected to increased pressure without the withdrawal of heat, this 
can only be effected by some free heat becoming latent; that is, some 
ice in the mixture must melt and be converted into water. In this is 
found the reason why mechanical pressure can influence the freezing 
point. You know that ice occupies more space than the water from 
which it is formed. When water freezes in closed vessels, it can 
burst not only glass vessels, but even iron shells. Inasmuch, there- 
fore, as in the compressed mixture of ice and water some of the ice 
melts and is converted into water, the volume of the mass diminishes, 
and the mass can yield more to the pressure upon it than it could 
have done without such an alteration of the freezing point. Pressure 
furthers in this case, as is usual in the interaction of various natural 
forces, the occurrence of a change, that is fusion, which is favourable 
to the development of its own activity. 

In Sir W. Thomson's experiments water and ice were confined in 
a closed vessel from which nothing could escape. The case is some- 
what different when, as with glaciers, the water disseminated in the 
compressed ice can escape through fissures. The ice is then com- 
pressed, but not the water which escapes. The compressed ice 
becomes colder in conformity with the lowering of its freezing point 
by pressure; but the freezing point of water which is not compressed 
is not lowered. Thus under these circumstances we have ice colder 
than 0° in contact with water at 0°. The consequence is that around 
the compressed ice water continually freezes and forms new ice, 
while on the other hand part of the compressed ice melts. 

This occurs, for instance, when only two pieces of ice are pressed 
against each other. By the water which freezes at their surfaces of 
contact they are firmly joined into one coherent piece of ice. With 
powerful pressure, and the chilling therefore great, this is quickly 
effected; but even with a feeble pressure it takes place, if sufficient 
time be given. Faraday, who discovered this property, called it the 
regelation of ice; the explanation of this phenomenon has been much 
controverted; I have detailed to you that which I consider most 
satisfactory. 

This freezing together of two pieces of ice is very readily effected 
by pieces of any shape, which must not, however, be at a lower 
temperature than 0°, and the experiment succeeds best when the 



234 HELMHOLTZ 

pieces are already in the act of melting.' They need only be strongly 
pressed together for a few minutes to make them adhere. The more 
plane are the surfaces in contact, the more complete is their union. 
But a very slight pressure is sufficient if the two pieces are left in 
contact for some time.^ 

This property of melting ice is also utilised by boys in making 
snowballs and snowmen. It is well known that this only succeeds 
either when the snow is already melting, or at any rate is only so 
much lower than o° that the warmth of the hand is sufficient to 
raise it to this temperature. Very cold snow is a dry loose powder 
which does not stick together. 

The process which children carry out on a small scale in making 
snowballs takes place in glaciers on the very largest scale. The 
deeper layers of what was originally line loose neve are compressed 
by the huge masses resting on them, often amounting to several 
hundred feet, and under this pressure they cohere with an ever 
firmer and closer structure. The freshly fallen snow originally con- 
sisted of delicate microscopically fine ice spicules, united and forming 
delicate six-rayed, feathery stars of extreme beauty. As often as the 
upper layers of the snow fields are exposed to the sun's rays, some 
of the snow melts; water permeates the mass, and on reaching the 
lower layers of still colder snow, it again freezes; thus it is that the 
firn first becomes granular and acquires the temperature of the 
freezing point. But as the weight of the superincumbent masses of 
snow continually increases by the firmer adherence of its individual 
granules, it ultimately changes into a dense and perfectly hard mass. 

This transformation of snow into ice may be artificially effected 
by using a corresponding pressure. 

We have here (Fig. 113) a cylindrical cast iron vessel, A A; the 
base, B B, is held by three screws, and can be detached, so as to 
remove the cylinder of ice which is formed. After the vessel has lain 
for a while in ice water, so as to reduce it to the temperature of 0°, 
it is packed full of snow, and then the cylindrical plug, C C, which 
fits the inner aperture, but moves in it with gentle friction, is forced 

' In the Lecture a series of small cylinders of ice, which had been prepared by a 
method to be afterwards described, were pressed with their plane ends against each 
other, and thus a cylindrical bar of ice produced. 

^ Vide the additions at the end of this Lecture. 



ICE AND GLACIERS 235 

in with the aid of an hydraulic press. The press used was such that 

the pressure to which the snow was exposed could be increased to 

fifty atmospheres. Of course the looser snow contracts to a very 

small volume under such a powerful pressure. The pressure is 

removed, the cylindrical plug 

taken out, the hollow again filled 

up with snow, and the process re- 

pea,ted until the entire form is 

filled with the mass of ice, which 

no longer gives way to pressure. 

The compressed snow which I 

now take out, you will see, has 

been transformed into a hard, 

angular, and translucent cylinder 

of ice; and how hard it is appears 

from the crash which ensues when 

I throw it to the ground. Just as 

the loose snow in the glaciers is 

pressed together to solid ice, so 

also in many places ready-formed 

irregular pieces of ice are joined 

and form clear and compact ice. 

This is most remarkable at the 

base of the glacier cascades. These 

are glacier falls where the upper 

part of the glacier ends at a steep rocky wall, and blocks of ice shoot 

down as avalanches over the edge of this wall. The heap of shattered 

blocks of ice which accumulate become joined at the foot of the 

rock-wall to a compact, dense mass, which then continues its way 

downwards as glacier. More frequent than such cascades, where 

the glacier-stream is quite dissevered, are places where the base of 

the valley has a steeper slope, as, for instance, the places in the Mer 

de Glace (Fig. 105), at g, of the Cascade of the Glacier du Geant, 

and at i and h of the great terminal cascade of the Glacier des Bois. 

The ice splits there into thousands of banks and clifJs, which then 

recombine towards the bottom of the steeper slope and form a 

coherent mass. 




Fig. 113 



236 HELMHOLTZ 

This also we may imitate in our ice mould. Instead of the snow 
I take irregular pieces of ice, press them together; add new pieces of 
ice, press them again, and so on, until the mould is full. When the 
mass is taken out it forms a compact coherent cylinder of tolerably 
clear ice, which has a perfectly sharp edge, and is an accurate copy 
of the mould. 

This experiment, which was first made by Tyndall, shows that a 
block of ice may be pressed into any mould just like a piece of wax. 
It might, perhaps, be thought that such a block had, by the pressure 
in the interior, been first reduced to powder so fine that it readily 
penetrated every crevice of the mould, and then that this powdered 
ice, like snow, was again combined by freezing. This suggests itself 
the more readily, since while the press is being worked a continual 
creaking and cracking is heard in the interior of the mould. Yet the 
mere aspect of the cylinders pressed from blocks of ice shows us that 
it has not been formed in this manner; for they are generally clearer 
than the ice which is produced from snow, and the individual larger 
pieces of ice which have been used to produce them are recognised, 
though they are somewhat changed and flattened. This is most 
beautiful when clear pieces of ice are laid in the form and the rest 
of the space stuffed full of snow. The cylinder is then seen to con- 
sist of alternate layers of clear and opaque ice, the former arising 
from the pieces of ice, and the latter from the snow; but here also 
the pieces of ice seem pressed into flat discs. 

These observations teach, then, that ice need not be completely 
smashed to fit into the prescribed mould, but that it may give way 
without losing its coherence. This can be still more completely 
proved, and we can acquire a still better insight into the cause of the 
pliability of ice, if we press the ice between two plane wooden boards, 
instead of in the mould, into which we cannot see. 

I place first an irregular cylindrical piece of natural ice, taken from 
the frozen surface of the river, with its two plane terminal surfaces 
between the plates of the press. If I begin to work, the block is 
broken by pressure; every crack which forms extends through the 
entire mass of the block; this splits into a heap of larger fragments, 
which again crack and are broken the more the press is worked. If 
the pressure is relaxed, all these fragments are, indeed, reunited by 



ICE AND GLACIERS 237 

freezing, but the aspect of the whole indicates that the shape of the 
block has resulted less from pliability than from fracture, and that 
the individual fragments have completely altered their mutual 
positions. 

The case is quite different when one of the cylinders which we 
have formed from snow or ice is placed between the plates of the 





Fig. 114 Fig. 115 

press. As the press is worked the creaking and cracking is heard, but 
it does not break; it gradually changes its shape, becomes lower and 
at the same time thicker; and only when it has been changed into a 
tolerably flat circular disc does it begin to give way at the edges and 
form cracks, like crevasses on a small scale. Fig. 114 shows the height 
and diameter of such a cylinder in its original condition; Fig. 115 
represents its appearance after the action of the press. 

A still stronger proof of the pliability of ice is afforded when one 
of our cylinders is forced through a narrow aperture. With this 
view I place a base on the previously described mould, which has a 
conical perforation, the external aperture of which is only two thirds 
the diameter of the cylindrical aperture of the form. Fig. 116 gives 
a section of the whole. If now I insert into this one of the com- 
pressed cylinders of ice, and force down the plug a, the ice is forced 
through the narrow aperture in the base. It at first emerges as a 
solid cylinder of the same diameter as the aperture; but as the ice 
follows more rapidly in the centre than at the edges, the free terminal 



238 HELMHOLTZ 

surface of the cylinder becomes curved, the end thickens, so that it 
could not be brought back through the aperture, and it ultimately 

splits off. Fig. 117 exhibits a series 
of shapes which have resulted in this 
manner.' 

Here also the cracks in the emerg- 
ing cylinder of ice exhibit a surpris- 
ing similarity with the longitudinal 
rifts which divide a glacier current 
where it presses through a narrow 
rocky pass into a wider valley. 

In the cases which we have de- 
scribed we see the change in shape of 
the ice taking place before our eyes, 
whereby the block of ice retains its 
coherence without breaking into individual pieces. The brittle mass 
of ice seems rather to yield like a piece of wax. 

A closer inspection of a clear cylinder of ice compressed from clear 
pieces of ice, while the pressure is being applied, shows us what takes 
place in the interior; for we then see an innumerable quantity of 
extremely fine radiating cracks shoot through it like a turbid cloud, 
which mostly disappear, though not completely, the moment the 




Fig. 116 





a 



Fig. 117 

pressure is suspended. Such a compressed block is distinctly more 
opaque immediately after the experiment than it was before; and 
the turbidity arises, as may easily be observed by means of a lens, 
from a great number of whitish capillary lines crossing the interior 

'In this experiment the lower temperature of the compressed ice sometimes 
extended so far through the iron form, that the water in the slit between the base 
plate and the cylinder froze and formed a thin sheet of ice, although the pieces of 
ice as well as the iron mould had previously laid in ice water, and could not be 
colder than 0°. 



ICE AND GLACIERS 239 

of the mass of what is otherwise clear. These Unes are the optical 
expression of extremely fine cracks^ which interpenetrate the mass 
of the ice. Hence we may conclude that the compressed block is 
traversed by a great number of fine cracks and fissures which render 
it pliable; that its particles become a little dispersed, and are there- 
fore withdrawn from pressure, and that immediately afterwards the 
greater part of the fissures disappear, owing to their sides freezing. 
Only in those places in which the surfaces of the small displaced 
particles do not accurately fit to each other some fissured spaces 
remain open, and are discovered as white lines and surfaces by the 
reflection of the light. 

These cracks and lamina also become more perceptible when the 
ice — which, as I before mentioned, is below zero immediately after 
pressure has been applied — is again raised to this temperature and 
begins to melt. The crevices then fill with water, and such ice then 
consists of a quantity of minute granules from the size of a pin's 
head to that of a pea, which are closely pushed into one another at 
the edges and projections, and in part have coalesced, while the nar- 
row fissures between them are full of water. A block of ice thus 
formed of ice granules adheres firmly together; but if particles be 
detached from its corners they are seen to consist of these angular 
granules. Glacier ice, when it begins to melt, is seen to possess the 
same structure, except that the pieces of which it consists are mostly 
larger than in artificial ice, attaining the size of a pigeon's egg. 

Glacier ice and compressed ice are thus seen to be substances of a 
granular structure, in opposition to regularly crystallised ice, such as 
is formed on the surface of still water. We here meet with the same 
differences as between calcareous spar and marble, both of which 
consist of carbonate of lime; but while the former is in large, regular 
crystals, the latter is made up of irregularly agglomerated crystalline 
grains. In calcareous spar, as well as in crystallised ice, the cracks 

* These cracks are probably quite empty and free from air, for they are also formed 
when perfectly clear and air-free pieces of ice are pressed in the form which has 
been previously filled with water, and where, therefore, no air could gain access to 
the pieces of ice. That such air-free crevices occur in glacier ice has been already 
demonstrated by Tyndall. When the compressed ice afterwards melts, these crevices 
fill up with water, no air being left. They are then, however, far less visible, and the 
whole block is therefore clearer. And just for this reason they could not originally 
have been filled with water. 



240 HELMHOLTZ 

produced by inserting the point o£ a knife extend through the mass, 
while in granular ice a crack which arises in one of the bodies where 
it must yield does not necessarily spread beyond the limits of the 
granule. 

Ice which has been compressed from snow, and has thus from the 
outset consisted of innumerable very fine crystalline needles, is seen 
to be particularly plastic. Yet in appearance it materially differs from 
glacier ice, for it is very opaque, owing to the great quantity of air 
which was originally inclosed in the flaky mass of snow, and which 
remains there as extremely minute bubbles. It can be made clearer 
by pressing a cylinder of such ice between wooden boards; the air 
bubbles appear then on the top of the cylinder as a light foam. If 
the discs are again broken, placed in the mould, and pressed into a 
cylinder, the air may gradually be more and more eliminated, and 
the ice be made clearer. No doubt in glaciers the originally whitish 
mass of neve is thus gradually transformed into the clear, trans- 
parent ice of the glacier. 

Lastly, when streaked cylinders of ice formed from pieces of snow 
and ice are pressed into discs, they become finely streaked, for both 
their clear and their opaque layers are uniformly extended. 

Ice thus striated occurs in numerous glaciers, and is no doubt 
caused, as Tyndall maintains, by snow falling between the blocks 
of ice; this mixture of snow and clear ice is again compressed in the 
subsequent path of the glacier, and gradually stretched by the motion 
of the mass: a process quite analogous to the artificial one which we 
have demonstrated. 

Thus to the eye of the natural philosopher the glacier, with its 
wildly heaped ice blocks, its desolate, stony, and muddy surface, and 
its threatening crevasses, has become a majestic stream whose peace- 
ful and regular flow has no parallel; which according to fixed and 
definite laws, narrows, expands, is heaped up, or, broken and shat- 
tered, falls down precipitous heights. If we trace it beyond its 
termination we see its waters, uniting to a copious brook, burst 
through its icy gate and flow away. Such a brook, on emerging from 
the glacier, seems dirty and turbid enough, for it carries away as 
powder the stone which the glacier has ground. We are disenchanted 
at seeing the wondrously beautiful and transparent ice converted 



ICE AND GLACIERS 24 1 

into such muddy water. But the water o£ the glacier streams is as 
pure and beautiful as the ice, though its beauty is for the moment 
concealed and invisible. We must search for these waters after they 
have passed through a lake in which they have deposited this 
powdered stone. The Lakes of Geneva, of Thun, of Lucerne, of 
Constance, the Lago Maggiore, the Lake of Como, and the Lago 
di Garda are chiefly fed with glacier waters; their clearness and their 
wonderfully beautiful blue or blue-green colour are the delight of 
all travellers. 

Yet, leaving aside the beauty of these waters, and considering only 
their utility, we shall have still more reason for admiration. The 
unsightly mud which the glacier streams wash away forms a highly 
fertile soil in the places where it is deposited; for its state of mechani- 
cal division is extremely fine, and it is moreover an utterly unex- 
hausted virgin soil, rich in the mineral food of plants. The fruitful 
layers of fine loam which extend along the whole Rhine plain as far 
as Belgium, and are known as Loess, are nothing more than the 
dust of ancient glaciers. 

Then, again, the irrigation of a district, which is effected by the 
snow fields and glaciers of the mountains, is distinguished from that 
of other places by its comparatively greater abundancy, for the moist 
air which is driven over the cold mountain peaks deposits there most 
of the water it contains in the form of snow. In the second place, the 
snow melts most rapidly in summer, and thus the springs which flow 
from the snow fields are most abundant in that season of the year 
in which they are most needed. 

Thus we ultimately get to know the wild, dead ice wastes from 
another point of view. From them trickles in thousands of rills, 
springs, and brooks the fructifying moisture which enables the 
industrious dwellers of the Alps to procure succulent vegetation and 
abundance of nourishment from the wild mountain slopes. On the 
comparatively small surface of the Alpine chain they produce the 
mighty streams the Rhine, the Rhone, the Po, the Adige, the Inn, 
which for hundreds of miles form broad, rich river valleys, extending 
through Europe to the German Ocean, the Mediterranean, the 
Adriatic, and the Black Sea. Let us call to mind how magnificently 
Goethe, in "Mahomet's Song," has depicted the course of the rocky 



242 HELMHOLTZ 

spring, from its origin beyond the clouds to its union with Father 
Ocean. It would be presumptuous after him to give such a picture 
in other than his own words: — 

And along, in triumph rolling, 
Names he gives to regions; cities 
Grow amain beneath his feet. 

On and ever on he rushes; 
Spire and turret fiery crested 
Marble palaces, the creatures 
Of his wealth, he leaves behind. 

Pine-buUt houses bears the Atlas 
On his giant shoulders. O'er his 
Head a thousand pennons rusde. 
Floating far upon the breezes. 
Tokens of his majesty. 

And so beareth he his brothers, 
And his treasures, and his children. 
To their primal sire expectant, 
All his bosom throbbing, heaving, 
With a wild tumultuous joy. 

Theodore Martin's Translation. 



ADDITIONS 

The theory of the regelation of ice has led to scientific discussions 
between Faraday and Tyndall on the one hand, and James and Sir 
W. Thomson on the other. In the text I have adopted the theory of 
the latter, and must now accordingly defend it. 

Faraday's experiments show that a very slight pressure, not more 
than that produced by the capillarity of the layer of water between 
two pieces of ice, is sufficient to freeze them together. James Thom- 
son observed that in Faraday's experiments pressure which could 
freeze them together was not utterly wanting. I have satisfied myself 
by my own experiments that only very slight pressure is necessary. It 
must, however, be remembered that the smaller the pressure the 
longer will be the time required to freeze the two pieces, and that 
then the junction will be very narrow and very fragile. Both these 
points are readily explicable on Thomson's theory. For under a 
feeble pressure the difference in temperature between ice and water 
will be very small, and the latent heat will only be slowly abstracted 
from the layers of water in contact with the pressed parts of the ice, 
so that a long time is necessary before they freeze. We must further 
take into account that we cannot in general consider that the two sur- 
faces are quite in contact; under a feeble pressure which does not ap- 
preciably alter their shape, they will only touch in what are practically 
three points. A feeble total pressure on the pieces of ice concentrated 
on such narrow surfaces will always produce a tolerably great local 
pressure under the influence of which some ice will melt, and the 
water thus formed will freeze. But the bridge which joins them will 
never be otherwise than narrow. 

Under stronger pressure, which may more completely alter the 
shape of the pieces of ice, and fit them against each other, and which 
will melt more of the surfaces that are first in contact, there will be 
a greater difference between the temperature of the ice and water, 
and the bridges will be more rapidly formed, and be of greater 
extent. 

243 



^^^ t±t:,L,SS/Ltl{JL,ll, 

In order to show the slow action of the small differences of tem- 
perature which here come into play, I made the following experi- 
ments. 

A glass flask with a drawn-out neck was half filled with water, 
which was boiled until all the air in the flask was driven out. The 
neck of the flask was then hermetically sealed. When cooled, the 
flask was void of air, and the water within it freed from the pressure 
of the atmosphere. As the water thus prepared can be cooled con- 
siderably below 0° C. before the first ice is formed, while when ice 
is in the flask it freezes at o° C, the flask was in the first instance 
placed in a freezing mixture until the water was changed into ice. It 
was afterwards permitted to melt slowly in a place the temperature 
of which was -\-2° C, until the half of it was liquefied. 

The flask thus half filled with water, having a disc of ice swim- 
ming upon it, was placed in a mixture of ice and water, being quite 
surrounded by the mixture. After an hour, the disc within the flask 
was frozen to the glass. By shaking the flask the disc was liberated, 
but it froze again. This occurred as often as the shaking was 
repeated. 

The flask was permitted to remain for eight days in the mixture, 
which was kept throughout at a temperature of o° C. During this 
time a number of very regular and sharply defined ice crystals were 
formed, and augmented very slowly in size. This is perhaps the 
best method of obtaining beautifully formed crystals of ice. 

While, therefore, the outer ice which had to support the pres- 
sure of the atmosphere slowly melted, the water within the flask, 
whose freezing point, on account of a defect of pressure, was 0.0075° 
C. higher, deposited crystals of ice. The heat abstracted from the 
water in this operation had, moreover, to pass through the glass of 
the flask, which, together with the small difference of temperature, 
explains the slowness of the freezing process. 

Now as the pressure of one atmosphere on a square millimetre 
amounts to about ten grammes, a piece of ice weighing ten grammes, 
which lies upon another and touches it in three places, the total 
surface of which is a square millimetre, will produce on these sur- 
faces a pressure of an atmosphere. Ice will therefore be formed more 
rapidly in the surrounding water than it was in the flask, where the 



ICE AND GLACIERS 245 

side of the glass was interposed between the ice and the water. Even 
with a much smaller weight the same result will follow in the course 
of an hour. The broader the bridges become, owing to the freshly 
formed ice, the greater will be the surfaces over which the pressure 
exerted by the upper piece of ice is distributed, and the feebler it 
will become; so that with such feeble pressure the bridges can only 
slowly increase, and therefore they will be readily broken when we 
try to separate the pieces. 

It cannot, moreover, be doubted that in Faraday's experiments, in 
which two perforated discs of ice were placed in contact on a hori- 
zontal glass rod, so that gravity exerted no pressure, capillary at- 
traction is sufficient to produce a pressure of some grammes between 
the plates, and the preceding discussions show that such a pressure, 
if adequate time be given, can form bridges between the plates. 

If, on the other hand, two of the above-described cylinders of ice 
are powerfully pressed together by the hands, they adhere in a few 
minutes so firmly that they can only be detached by the exertion of 
a considerable force, for which indeed that of the hands is some- 
times inadequate. 

In my experiments I found that the force and rapidity with which 
the pieces of ice united were so entirely proportional to the pressure 
that I cannot but assign this as the actual and sufficient cause of 
their union. 

In Faraday's explanation, according to which regelation is due 
to a contact action of ice and water, I find a theoretical difficulty. By 
the water freezing, a considerable quantity of latent heat must be set 
free, and it is not clear what becomes of this. 

Finally, if ice in its change into water passes through an inter- 
mediate viscous condition, a mixture of ice and water which was 
kept for days at a temperature of 0° must ultimately assume this 
condition in its entire mass, provided its temperature was uniform 
throughout; this however is never the case. 

As regards what is called the plasticity of ice, James Thomson has 
given an explanation of it in which the formation of cracks in the 
interior is not presupposed. No doubt when a mass of ice in dif- 
ferent parts of the interior is exposed to different pressures, a por- 
tion of the more powerfully compressed ice will melt; and the latent 



246 HELMHOLTZ 

heat necessary for this will be supplied by the ice which is less 
strongly compressed, and by the water in contact with it. Thus ice 
would melt at the compressed places, and water would freeze in 
those which are not pressed: ice would thus be gradually transformed 
and yield to pressure. It is also clear that, owing to the very small 
conductivity for heat which ice possesses, a process of this kind must 
be extremely slow, if the compressed and colder layers of ice, as in 
glaciers, are at considerable distances from the less compressed ones, 
and from the water which furnishes the heat for melting. 

To test this hypothesis, I placed in a cylindrical vessel, between two 
discs of ice of three inches in diameter, a smaller cylindrical piece of 
an inch in diameter. On the uppermost disc I placed a wooden disc, 
and this I loaded with a weight of twenty pounds. The section of 
the narrow piece was thus exposed to a pressure of more than an 
atmosphere. The whole vessel was packed between pieces of ice, and 
left for five days in a room the temperature of which was a few 
degrees above the freezing point. Under these circumstances the 
ice in the vessel, which was exposed to the pressure of the weight, 
should melt, and it might be expected that the narrow cylinder on 
which the pressure was most powerful should have been most 
melted. Some water was indeed formed in the vessel, but mostly at 
the expense of the larger discs at the top and bottom, which being 
nearest the outside mixture of ice and water, could acquire heat 
through the sides of the vessel. A small welt, too, of ice, was formed 
round the surface of contact of the narrower with the lower broad 
piece, which showed that the water, which had been formed in con- 
sequence of the pressure, had again frozen in places in which the 
pressure ceased. Yet under these circumstances there was no appreci- 
able alteration in the shape of the middle piece which was most 
compressed. 

This experiment shows that although changes in the shape of the 
pieces of ice must take place in the course of time in accordance with 
J. Thomson's explanation, by which the more strongly compressed 
parts melt, and new ice is formed at the places which are freed from 
pressure, these changes must be extremely slow when the thickness 
of the pieces of ice through which the heat is conducted is at all con- 
siderable. Any marked change in shape by melting in a medium the 



ICE AND GLACIERS 247 

temperature of which is everywhere 0°, could not occur without 
access of external heat, or from the uncompressed ice and water; and 
with the small differences in temperature which here come into play, 
and from the badly conducting power of ice, it must be extremely 
slow. 

That on the other hand, especially in granular ice, the formation 
of cracks, and the displacement of the surfaces of those cracks, ren- 
der such a change of form possible, is shown by the above-described 
experiments on pressure; and that in glacier ice changes of form thus 
occur, follows from the banded structure, and the granular aggrega- 
tion which is manifest on melting, and also from the manner in 
which the layers change their position when moved, and so forth. 
Hence, I doubt not that Tyndall has discovered the essential and 
principal cause of the motion of glaciers, in referring it to the forma- 
tion of cracks and to regelation. 

I would at the same time observe that a quantity of heat, which 
is far from inconsiderable, must be produced by friction in the larger 
glaciers. It may be easily shown by calculation that when a mass of 
firn moves from the Col du Geant to the source of the Arveyron, the 
heat due to the mechanical work would be sufficient to melt a four- 
teenth part of the mass. And as the friction must be greatest in those 
places that are most compressed, it will at any rate be sufficient to 
remove just those parts of the ice which offer most resistance to 
motion. 

I will add, in conclusion, that the above-described granular struc- 
ture of ice is beautifully shown in polarised light. If a small clear 
piece is pressed in the iron mould, so as to form a disc of about five 
inches in thickness, this is sufficiently transparent for investigation. 
Viewed in the polarising apparatus, a great number of variously 
coloured small bands and rings are seen in the interior; and by the 
arrangement of their colours it is easy to recognise the limits of the 
ice granules, which, heaped on one another in irregular order of 
their optical axes, constitute the plate. The appearance is essentially 
the same when the plate has just been taken out of the press, and 
the cracks appear in it as whitish lines, as afterwards when these 
crevices have been filled up in consequence of the ice beginning to 
melt. 



248 HELMHOLTZ 

In order to explain the continued coherence of the piece of ice 
during its change of form, it is to be observed that in general the 
cracks in the granular ice are only superficial, and do not extend 
throughout its entire mass. This is directly seen during the pressing 
of the ice. The crevices form and extend in different directions, like 
cracks produced by a heated wire in a glass tube. Ice possesses a 
certain degree of elasticity, as may be seen in a thin flexible plate. A 
fissured block of ice of this kind will be able to undergo a displace- 
ment at the two sides which form the crack, even when these con- 
tinue to adhere in the unfissured part of the block. If then part of 
the fissure at first formed is closed by regelation, the fissure can 
extend in the opposite direction without the continuity of the block 
being at any time disturbed. It seems to me doubtful, too, whether 
in compressed ice and in glacier ice, which apparently consists of 
interlaced polyhedral granules, these granules, before any attempt is 
made to separate them, are completely detached from each other, and 
are not rather connected by ice bridges which readily give way; and 
whether these latter do not produce the comparatively firm coherence 
of the apparent heap of granules. 

The properties of ice here described are interesting from a physical 
point of view, for they enable us to follow so closely the transition 
from a crystalline body to a granular one; and they give the causes 
of the alteration of its properties better than in any other well-known 
example. Most natural substances show no regular crystalline struc- 
ture; our theoretical ideas refer almost exclusively to crystallised and 
perfectly elastic bodies. It is precisely in this relationship that the 
transition from fragile and elastic crystalline ice into plastic granular 
ice is so very instructive. 



THE WAVE THEORY OF LIGHT 
THE TIDES 

BY 

SIR WILLIAM THOMSON 
(LORD KELVIN) 



INTRODUCTORY NOTE 

William Thomson, Baron Kelvin of Largs, was born in Belfast, Ire- 
land, June 24, 1824. He was the son of the professor of mathematics at 
Glasgow University, and himself entered that institution at the age of 
eleven. By the time he was twenty-one he graduated from Cambridge 
as Second Wrangler, and, after studying in Paris, he returned to Scotland 
to become, as professor of natural philosophy, the colleague of his father 
and elder brother. The story of his life thenceforth is the record of 
amazingly brilliant and fruitful scientific work, recognized by the award 
of almost all the honors appropriate to such service, from learned societies, 
universities, and governments at home and abroad. His part in laying the 
Adantic Cable was the occasion of his receiving knighthood, and in 1892 
he was raised to the peerage. He held his professorship at Glasgow for 
fifty-three years, and later was chosen its Chancellor. He died on Decem- 
ber 17, 1907, and was buried in Westminster Abbey. 

Lord Kelvin's activities were remarkable for both profundity and 
range. A large number of his results are to be appreciated only by the 
highly skilled mathematician and physicist; but his speculations on the 
ultimate constitution of matter; his statement of the principle of the dissi- 
pation of energy, with its bearing upon the age of life on the earth; his 
calculations as to the age of the earth itself, and much more, are of great 
general interest. His fertility in practical invention was no less notable. 
He contrived a large number of instruments; his services to navigation 
and ocean telegraphy being especially valuable. Long before his death 
he was recognized as the most distinguished man of science of his time 
and country, and he was also the most loved. 

The lectures which follow are favorable examples of his power of 
exposition in subjects in which he had no superior. 



THE WAVE THEORY OF 
LIGHT 

[A LECTURE DELIVERED AT THE ACADEMY OF MUSIC, 

PHILADELPHIA, UNDER THE AUSPICES OF 

THE FRANKLIN INSTITUTE, 

SEPTEMBER 29th, 1884.] 

THE subject upon which I am to speak to you this evening 
is happily for me not new in Philadelphia. The beautiful 
lectures on light which were given several years ago by Presi- 
dent Morton, of the Stevens' Institute, and the succession of lectures 
on the same subject so admirably illustrated by Professor Tyndall, 
which many now present have heard, have fully prepared you for 
anything I can tell you this evening in respect to the wave theory of 
light. 

It is indeed my humble part to bring before you only some mathe- 
matical and dynamical details of this great theory. I cannot have 
the pleasure of illustrating them to you by anything comparable 
with the splendid and instructive experiments which many of you 
have already seen. It is satisfactory to me to know that so many 
of you, now present, are so thoroughly prepared to understand any- 
thing I can say, that those who have seen the experiments will not 
feel their absence at this time. At the same time I wish to make 
them intelligible to those who have not had the advantages to be 
gained by a systematic course of lectures. -I must say, in the first 
place, without further preface, as time is short and the subject is 
long, simply that sound and light are both due to vibrations propa- 
gated in the manner of waves; and I shall endeavour in the first 
place to define the manner of propagation and the mode of motion 
that constitute those two subjects of our senses, the sense of sound 
and the sense of light. 

251 



252 KELVIN 

Each is due to vibrations, but the vibrations of Ught differ widely 
from the vibrations of sound. Something that I can tell you more 
easily than anything in the way of dynamics or mathematics re- 
specting the two classes of vibrations is, that there is a great difference 
in the frequency of the vibrations of light when compared with the 
frequency of the vibrations of sound. The term "frequency" applied 
to vibrations is a convenient term, appUed by Lord Rayleigh in his 
book on sound to a definite number of full vibrations of a vibrating 
body per unit of time. Consider, then, in respect to sound, the fre- 
quency of the vibrations of notes, which you all know in music 
represented by letters, and by the syllables for singing, the do, re, mi, 
&c. The notes of the modern scale correspond to different frequencies 
of vibrations. A certain note and the octave above it, correspond to 
a certain number of vibrations per second, and double that number. 

I may conveniently explain in the first place the note called 
'C; I mean the middle 'C; I believe it is the C of the tenor voice, 
that most nearly approaches the tones used in speaking. That note 
corresponds to two hundred and fifty-six full vibrations per sec- 
ond — two hundred and fifty-six times to and fro per second 
of time. 

Think of one vibration per second of time. The seconds pendulum 
of the clock performs one vibration in two seconds, or a half vibration 
in one direction per second. Take a ten-inch pendulum of a drawing- 
room clock, which vibrates twice as fast as the pendulum of an 
ordinary eight-day clock, and it gives a vibration of one per second, 
a full period of one per second to and fro. Now think of three vibra- 
tions per second. I can move my hand three times per second easily, 
and by a violent effort I can move it to and fro five times per second. 
With four times as great force, if I could apply it, I could move it 
twice five times per second. 

Let us think, then, of an exceedingly muscular arm that would 
cause it to vibrate ten times per second, that is, ten times to the 
left and ten times to the right. Think of twice ten times, that is, 
twenty times per second, which would require four times as much 
force; three times ten, or thirty times a second, would require nine 
times as much force. If a person were nine times as strong as the 
most muscular arm can be, he could vibrate his hand to and fro thirty 



THE WAVE THEORY OF LIGHT 253 

times per second, and without any other musical instrument could 
make a musical note by the movement of his hand which would 
correspond to one of the pedal notes of an organ. 

If you want to know the length of a pedal pipe, you can calculate 
it in this way. There are some numbers you must remember, and 
one of them is this. You, in this country, are subjected to the British 
insularity in weights and measures; you use the foot and inch and 
yard. I am obliged to use that system, but I apologise to you for 
doing so, because it is so inconvenient, and I hope all Americans 
will do everything in their power to introduce the French metrical 
system. I hope the evil action performed by an English minister 
whose name I need not mention, because I do not wish to throw 
obloquy on any one, may be remedied. He abrogated a useful rule, 
which for a short time was followed, and which I hope will soon be 
again enjoined, that the French metrical system be taught in all 
our national schools. I do not know how it is in America. The school 
system seems to be very admirable, and I hope the teaching of the 
metrical system will not be let slip in the American schools any 
more than the use of the globes. I say this seriously: I do not think 
any one knows how seriously I speak of it. I look upon our English 
system as a wickedly brain-destroying piece of bondage under which 
we suffer. The reason why we continue to use it is the imaginary 
difficulty of making a change, and nothing else; but I do not think 
in America that any such difficulty should stand in the way of 
adopting so splendidly useful a reform. 

I know the velocity of sound in feet per second. If I remember 
rightly, it is 1,089 ^^^t per second in dry air at the freezing tempera- 
ture, and 1,115 fss' psr second in air of what we would call moderate 
temperature, 59 or 60 degrees — (I do not know whether that tem- 
perature is ever attained in Philadelphia or not; I have had no ex- 
perience of it, but people tell me it is sometimes 59 or 60 degrees in 
Philadelphia, and I believe them) — in round numbers let us call 
the speed 1,000 feet per second. Sometimes we call it a thousand 
musical feet per second, it saves trouble in calculating the length of 
organ pipes; the time of vibration in an organ pipe is the time it 
takes a vibration to run from one end to the other and back. In an 
organ pipe 500 feet long the period would be one per second; in an 



254 KELVIN 

organ pipe ten feet long the period would be 50 per second; in 
an organ pipe twenty feet long the period would be 25 per second at 
the same rate. Thus 25 per second, and 50 per second of frequencies 
correspond to the periods of organ pipes of 20 feet and 10 feet. 

The period of vibration of an organ pipe, open at both ends, is 
approximately the time it takes sound to travel from one end to the 
other and back. You remember that the velocity in dry air in a pipe 
10 feet long is a little more than 50 periods per second; going up to 
256 f)eriods per second, the vibrations correspond to those of a pipe 
two feet long. Let us take 512 periods per second; that corresponds 
to a pipe about a foot long. In a flute, open at both ends, the holes 
are so arranged that the length of the sound-wave is about one foot, 
for one of the chief "open notes." Higher musical notes correspond 
to greater and greater frequency of vibration, viz., 1,000, 2,000, 4,000, 
vibrations per second; 4,000 vibrations per second correspond to a 
piccolo flute of exceedingly small length; it would be but one and 
a half inches long. Think of a note from a little dog-call, or other 
whistle, one and a half inches long, open at both ends, or from a little 
key having a tube three quarters of an inch long, closed at one end; 
you will then have 4,000 vibrations per second. 

A wave-length of sound is the distance traversed in the period 
of vibration. I will illustrate what the vibrations of sound are by this 
condensation travelling along our picture on the screen. Alternate 
condensations and rarefactions of the air are made continuously by 
a sounding body. When I pass my hand vigorously in one direction, 
the air before it becomes dense, and the air on the other side becomes 
rarefied. When I move it in the other direction these things become 
reversed; there is a spreading out of condensation from the place 
where my hand moves in one direction and then in the reverse. Each 
condensation is succeeded by a rarefaction. Rarefaction succeeds 
condensation at an interval of one-half what we call "wave-lengths." 
Condensation succeeds condensation at the full interval of a wave- 
length. 

We have here these luminous particles on this scale,' representing 
portions of air close together, more dense; a little higher up, por- 

' Alluding to a moving diagram of wave motion of sound produced by a working 
slide for lantern projection. 



THE WAVE THEORY OF LIGHT 255 

tions of air less dense. I now slowly turn the handle of the apparatus 
in the lantern, and you see the luminous sectors showing condensa- 
tion travelling slowly upwards on the screen; now you have another 
condensation making one wave-length. 

This picture or chart represents a wave-length of four feet. It 
represents a wave of sound four feet long. The fourth part of a 
thousand is 250. What we see now of the scale represents the lower 
note C of the tenor voice. The air from the mouth of a singer is 
alternately condensed and rarefied just as you see here. But that 
process shoots forward at the rate of about one thousand feet per 
second; the exact period of the motion being 256 vibrations per 
second for the actual case before you. 

Follow one particle of the air forming part of a sound wave, as 
represented by these moving spots of light on the screen; now it goes 
down, then another portion goes down rapidly; now it stops going 
down; now it begins to go up; now it goes down and up again. As 
the maximum of condensation is approached it is going up with 
diminishing maximum velocity. The maximum of rarefaction has 
now reached it, and the particle stops going up and begins to move 
down. When it is of mean density the particles are moving with 
maximum velocity, one way or the other. You can easily follow these 
motions, and you will see that each particle moves to and fro and 
the thing that we call condensation travels along. 

I shall show the distinction between these vibrations and the vi- 
brations of light. Here is the fixed appearance of the particles when 
displaced but not in motion. You can imagine particles of some- 
thing, the thing whose motion constitutes light. This thing we call 
the luminiferous ether. That is the only substance we are confident 
of in dynamics. One thing we are sure of, and that is the reality 
and substantiality of the luminiferous ether. This instrument is 
merely a method of giving motion to a diagram designed for the 
purpose of illustrating wave motion of light. I will show you the 
same thing in a fixed diagram, but this arrangement shows the 
mode of motion. 

Now follow the motion of each particle. This represents a particle 
of the luminiferous ether, moving at the greatest speed when it is 
at the middle position. 



256 KELVIN 

You see the two modes of vibration,^ sound and light now moving 
together; the travelUng of the wave of condensation and rarefaction, 
and the travelling of the wave of transverse displacement. Note 
the direction of propagation. Here it is from your left to your 
right, as you look at it. Look at the motion when made faster. We 
have now the direction reversed. The propagation of the wave is 
from right to left, again the propagation of the wave is from left 
to right; each particle moves perpendicularly to the line of propa- 
gation. 

I have given you an illustration of the vibration of sound waves, 
but I must tell you that the movement illustrating the condensation 
and rarefaction represented in that moving diagram are necessarily 
very much exaggerated, to let the motion be perceptible, whereas 
the greatest condensation in actual sound motion is not more than 
one or two per cent, or a small fraction of a per cent. Except that 
the amount of condensation was exaggerated in the diagram for 
sound, you have in the chart a correct representation of what actually 
takes place in sounding the low note C. 

On the other hand, in the moving diagram representing light 
waves what had we? We had a great exaggeration of the inclina- 
tion of the line of particles. You must first imagine a line of particles 
in a straight line, and then you must imagine them disturbed into 
a wave-curve, the shape of the curve corresponding to the disturb- 
ance. Having seen what the propagation of the wave is, look at this 
diagram and then look at that one. This, in light, corresponds to the 
different sounds I spoke of at first. The wave-length of light is the 
distance from crest to crest of the wave, or from hollow to hollow. 
I speak of crests and hollows, because we have a diagram of ups and 
downs as the diagram is placed. 

Here, then, you have a wave-length.' In this lower diagram (Fig. 
119) you have a wave-length of violet light. It is but one-half the 
length of the upper wave of red light; the period of vibration is but 
half as long. Now there, on an enormous scale, exaggerated not only 
as to slope, but immensely magnified as to wave-length, we have an 

^ showing two moving diagrams, simultaneously, on the screen, one depicting a 
wave motion of light, the other a sound vibration. 

' Exhibiting a large drawing, or chart, representing a red and a violet wave of light 
(reproduced in Figs. 118 and 119). 



THE WAVE THEORY OF LIGHT 257 

illustration of the waves of violet light. The drawing marked "red" 
(Fig. 118) corresponds to red light, and this lower diagram corre- 
sponds to violet light. The upper curve really corresponds to some- 
thing a Uttle below the red ray of light in the spectrum, and the 
lower curve to something beyond the violet light. The variation in 
wave-length between the most extreme rays is in the proportion of 
four and a half of red to eight of the violet, instead of four and 
eight; the red waves are nearly as one to two of the violet. 

To make a comparison between the number of vibrations for each 
wave of sound and the number of vibrations constituting light waves, 
I may say that 30 vibrations per second is about the smallest number 
which will produce a musical sound; 50 per second gives one of the 




Fig. 118 — Waves of Red Light 




Fig. 119 — ^Waves of Violet Light 



grave pedal notes of an organ, 100 or 200 per second give the low 
notes of the bass voice, higher notes with 250 per second, 300 per 
second, 1,000, 4,000 up to 8,000 per second give about the shrillest 
notes audible to the human ear. 

Instead of the numbers, which we have, say in the most com- 
monly used part of the musical scale, /. e., from 200 or 300 to 600 or 
700 per second, we have millions of millions of vibrations per second 
in Ught waves: that is to say, 400 per second, instead of 400 million 
million per second, which is the number of vibrations performed 
when we have red light produced. 

An exhibition of red light travelling through space from the 
remotest star is due to propagation by waves or vibrations, in which 
(each individual particle of the transmitting medium vibrates to and 
'fro 400 million million times in a second. 



258 KELVIN 

Some people say they cannot understand a million million. Those 
people cannot understand that twice two makes four. That is the 
way I put it to people who talk to me about the incomprehensibility 
of such large numbers. I say finitude is incomprehensible, the infi- 
nite in the universe is comprehensible. Now apply a little logic to 
this. Is the negation of infinitude incomprehensible? What would 
you think of a universe in which you could travel one, ten, or a thou- 
sand miles, or even to California, and then find it come to an end? 
Can you suppose an end of matter or an end of space ? The idea is 
incomprehensible. Even if you were to go millions and millions of 
miles the idea of coming to an end is incomprehensible. You can 
understand one thousand per second as easily as you can understand 
one per second. You can go from one to ten, and ten times ten and 
then to a thousand without taxing your understanding and then you 
can go on to a thousand million and a million million. You can all 
understand it. 

Now 400 million million vibrations per second is the kind of 
thing that exists as a factor in the illumination by red light. Violet 
light, after what we have seen and have had illustrated by that curve 
(Fig. 119), I need not tell you corresponds to vibrations of about 
800 million million per second. There are recognisable qualities of 
light caused by vibrations of much greater frequency and much less 
frequency than this. You may imagine vibrations having about twice 
the frequency of violet light, and others having about one-fifteenth 
the frequency of red light, and still you do not pass the limit of the 
range of continuous phenomena, only a part of which constitutes 
visible light. 

When we go below visible red light what have we? We have 
something we do not see with the eye, something that the ordinary 
photographer does not bring out on his photographically sensitive 
plates. It is light, but we do not see it. It is something so closely 
continuous with visible light, that we may define it by the name of 
invisible light. It is commonly called radiant heat; invisible radiant 
heat. Perhaps, in this thorny path of logic, with hard words flying in 
our faces, the least troublesome way of speaking of it is to call it 
radiant heat. The heat effect you experience when you go near a 
bright hot coal fire, or a hot steam boiler; or when you go near, but 



THE WAVE THEORY OF LIGHT 259 

not over, a set of hot water pipes used for heating a house; the thing 
we perceive in our faces and hands when we go near a boiling pot 
and hold the hand on a level with it, is radiant heat; the heat of the 
hands and face caused by a hot fire, or by a hot ketde when held 
under the kettle, is also radiant heat. 

You might readily make the experiment with an earthen teapot; 
it radiates heat better than polished silver. Hold your hands below 
the teapot and you perceive a sense of heat; above it you get more 
heat; either way you perceive heat. If held over the teapot you 
readily understand that there is a little current of hot air rising; 
if you put your hand under the teapot you find cold air rising, and 
the upper side of your hand is heated by radiation while the lower 
side is fanned and is actually cooled by virtue of the heated kettle 
above it. 

That perception by the sense of heat, is the perception of some- 
thing actually continuous with light. We have knowledge of rays 
of radiant heat perceptible down to (in round numbers) about four 
times the wave-length, or one-fourth the period, of visible or red 
light. Let us take red light at 400 million million vibrations per sec- 
ond, then the lowest radiant heat, as yet investigated, is about 100 
million million per second of frequency of vibration. 

I had hoped to be able to give you a lower figure. Professor 
Langley has made splendid experiments on the top of Mount Whit- 
ney, at the height of 15,000 feet above the sea-level, with his "Bolom- 
eter," and has made actual measurements of the wave-length of 
radiant heat down to exceedingly low figures. I will read you one of 
the figures; I have not got it by heart yet, because I am expecting 
more from him.* I learned a year and a half ago that the lowest 
radiant heat observed by the diffraction method of Professor Langley 
corresponds to 28 one hundred thousandths of a centimetre of wave- 
length, 28 as compared with red light, which is 7.3; or nearly four- 
fold. Thus wave-lengths of four times the amplitude, or one-fourth 

* Since my lecture I have heard from Professor Langley that he has measured the 
refrangibility by a rock salt prism, and inferred the wave-length of heat rays from a 
"Leslie cube" (a metal vessel filled with hot water and radiating heat from a blackened 
side). The greatest wave-length he has thus found is one-thousandth of a centimetre, 
which is seventeen times that of sodium light — the corresponding period being about 
thirty million million per second. 

November, 1884.— W. T. 



26o KELVIN 

the frequency per second of red light, have been experimented on 
by Professor Langley and recognised as radiant heat. 

Everybody knows the "photographer's hght," and has heard of 
invisible Hght producing visible effects upon the chemically pre- 
pared plate in the camera. Speaking in round numbers, I may say 
that, in going up to about twice the frequency I have mentioned for 
violet light, you have gone to the extreme end of the range of known 
light of the highest rates of vibration; I mean to say that you have 
reached the greatest frequency that has yet been observed. Photo- 
graphic, or actinic light, as far as our knowledge extends at present, 
takes us to a little less than one-half the wave-length of violet light. 

You will thus see that while our acquaintance with wave motion 
below the red extends down to one quarter of the slowest rate which 
affects the eye, our knowledge of vibrations at the other end of the 
scale only comprehends those having twice the frequency of violet 
light. In round numbers we have 4 octaves of light, corresponding 
to 4 octaves of sound in music. In music the octave has a range to a 
note of double frequency. In light we have one octave of visible 
light, one octave above the visible range and two octaves below the 
visible range. We have 100 per second, 200 per second, 400 per 
second (million million understood) for invisible radiant heat; 800 
per second for visible light, and 1,600 per second for invisible or 
actinic light. 

One thing common to the whole is the heat effect. It is extremely 
small in moonlight, so small that until recently nobody knew there 
was any heat in the moon's rays. Herschel thought it was perceptible 
in our atmosphere by noticing that it dissolved away very light 
clouds, an effect which seemed to show in full moonlight more than 
when we have less than full moon. Herschel, however, pointed this 
out as doubtful; but now, instead of its being a doubtful question, 
we have Professor Langley giving as a fact that the light from the 
moon drives the indicator of his sensitive instrument clear across the 
scale, showing a comparatively prodigious heating effect! 

I must tell you that if any of you want to experiment with the 
heat of the moonlight, you must measure the heat by means of 
apparatus which comes within the influence of the moon's rays only. 
This is a very necessary precaution; if, for instance, you should take 



THE WAVE THEORY OF LIGHT 261 

your Bolometer or other heat detector from a comparatively warm 
room into the night air, you would obtain an indication of a fall in 
temperature owing to this change. You must be sure that your 
apparatus is in thermal equilibrium with the surrounding air, then 
take your burning-glass, and first point it to the moon and then to 
space in the sky beside the moon; you thus get a differential measure- 
ment in which you compare the radiation of the moon with the 
radiation of the sky. You will then see that the moon has a distinctly 
heating effect. 

To continue our study of visible light, that is undulations extend- 
ing from red to violet in the spectrum (which I am going to show 



iiiiiiniiiiii|iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii|iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiigiiiii 
J I i I 



AaBCD Eb ¥ G H 

Fio. 120 — ^The Solar Spectrum 

you presently), I would first point out on this chart (Fig. 120) that 
in the section from letter A to letter D we have visual effect and 
heating effect only; but no ordinary chemical or photographic effect. 
Photographers can leave their usual sensitive chemically prepared 
plates exposed to yellow light and red light without experiencing any 
sensible effect; but when you get toward the blue end of the spectrum 
the photographic effect begins to tell, and more and more strongly 
as you get towards the violet end. When you get beyond the violet 
there is the invisible light known chiefly by its chemical action. From 
yellow to violet we have visual effect, heating effect, and chemical 
effect, all three; above the violet only chemical and heating effects, 
and so little of the heating effect that it is scarcely perceptible. 

The prismatic spectrum is Newton's discovery of the composition 
of white light. White light consists of every variety of colour from 
red to violet. Here, now, we have Newton's prismatic spectrum, pro- 
duced by a prism. I will illustrate a little in regard to the nature of 
colour by putting something before the light which is like coloured 
glass; it is coloured gelatin. I will put in a plate of red gelatin which 
is carefully prepared of chemical materials and see what that will do. 
Of all the light passing to it from violet to red it only lets through 
the red and orange, giving a mixed reddish colour. Here is a plate 



262 KELVIN 

of green gelatin: the green absorbs all the red, giving only green. 
Here is a plate absorbing something from each portion of the spec- 
trum, taking away a great deal of the violet and giving a yellow or 
orange appearance to the light. Here is another absorbing the green 
and all the violet, leaving red, orange, and a very little faint green. 

When the spectrum is very carefully produced, far more carefully 
than Newton knew how to show it, we have a homogeneous spec- 
trum. It must be noticed that Nevrton did not understand what we 
call a homogeneous spectrum; he did not produce it, and does not 
point out in his writings the conditions for producing it. With an 
exceedingly fine line of light we can bring it out as in sunlight, like 
this upper picture — red, orange, yellow, green, blue, indigo, and 
violet, according to Newton's nomenclature. Newton never used a 
narrow beam of light, and so could not have had a homogeneous 
spectrum. 

This is a diagram painted on glass and showing the colours as we 
know them. It would take two or three hours if I were to explain 
the subject of spectrum analysis to-night. We must tear ourselves 
away from it. I will just read out to you the wave-lengths corre- 
sponding to the different positions of the sun's spectrum of certain 
dark lines commonly called "Fraunhofer's lines." I will take as a 
unit the one hundred thousandth of a centimetre. A centimetre is .4 
of an inch; it is a rather small half an inch. I take the thousandth 
of a centimetre and the hundredth of that as a unit. At the red end 
of the spectrum the light in the neighbourhood of that black line A 
(Fig. 120) has for its wave-length 7.6; B has 6.87; D has 5.89; the 
"frequency" for A is 3.9 times 100 million million, the freauency of 
D light is 5.1 times 100 million million per second. 

Now what force is concerned in those vibrations as compared vwth 
sound at the rate of 400 vibrations per second? Suppose for a 
moment the same matter was to move to and fro through the same 
range but 400 million million times per second. The force required 
is as the square of the number expressing the frequency. Double 
frequency would require quadruple force for the vibration of the 
same body. Suppose I vibrate my hand again, as I did before. If I 
move it once per second a moderate force is required; for it to 
vibrate ten times per second 100 times as much force is required; for 



THE WAVE THEORY OF LIGHT 263 

400 vibrations per second 160,000 times as much force. If I move 
my hand once per second through a space of a quarter of an inch a 
very small force is required; it would require very considerable force 
to move it ten times a second, even through so small a range; but 
think of the force required to move a tuning fork 400 times a second, 
and compare that with the force required for a motion of 400 million 
million times a second. If the mass moved is the same, and the range 
of motion is the same, then the force would be one million million 
miUion million times as great as the force required to move the 
prongs of the tuning fork — it is as easy to understand that number 
as any number like 2, 3, or 4. Consider now what that number 
means and what we are to infer from it. What force is there in the 
space between my eye and that light? What forces are there in the 
space between our eyes and the sun, and our eyes and the remotest 
visible star? There is matter and there is motion, but what magni- 
tude of force may there be ? 

I move through this "luminiferous ether" as if it were nothing. 
But were there vibrations with such frequency in a medium of steel 
or brass, they would be measured by millions and millions and 
millions of tons' action on a square inch of matter. There are no 
such forces in our air. Comets make a disturbance in the air, and 
perhaps the luminiferous ether is split up by the motion of a comet 
through it. So when we explain the nature of electricity, we explain 
it by a motion of the luminiferous ether. We cannot say that it is 
electricity. What can this luminiferous ether be? It is something 
that the planets move through with the greatest ease. It permeates 
our air; it is nearly in the same condition, so far as our means of 
judging are concerned, in our air and in the inter-planetary space. 
The air disturbs it but little; you may reduce air by air-pumps to the 
hundred thousandth of its density, and you make little effect in the 
transmission of light through it. The luminiferous ether is an elastic 
solid, for which the nearest analogy I can give you is this jelly which 
you see,' and the nearest analogy to the waves of light is the motion, 
which you can imagine, of this elastic jelly, with a ball of wood 
floating in the middle of it. Look there, when with my hand I 

' Exhibiting a large bowl of clear jelly with a srr\all red wooden ball embedded 
in the surface near the centre. 



264 KELVIN 

vibrate the little red ball up and down, or when I turn it quickly 
round the vertical diameter, alternately in opposite directions; — that 
is the nearest representation I can give you of the vibrations of lumi- 
niferous ether. 

Another illustration is Scottish shoemakers' wax or Burgundy 
pitch, but I know Scottish shoemakers' wax better. It is heavier than 
water, and absolutely answers my purpose. I take a large slab of the 
wax, place it in a glass jar filled with water, place a number of corks 
on the lower side and bullets on the upper side. It is brittle like the 
Trinidad pitch or Burgundy pitch which I have in my hand — you 
can see how hard it is — but when left to itself it flows like a fluid. 
The shoemakers' wax breaks with a brittle fracture, but it is viscous 
and gradually yields. 

What we know of the luminiferous ether is that it has the rigidity 
of a solid and gradually yields. Whether or not it is brittle and cracks 
we cannot yet tell, but I believe the discoveries in electricity and the 
motions of comets and the marvellous spurts of light from them, tend 
to show cracks in the luminiferous ether — show a correspondence 
between the electric flash and the aurora borealis and cracks in the 
luminiferous ether. Do not take this as an assertion, it is hardly more 
than a vague scientific dream: but you may regard the existence of 
the luminiferous ether as a reality of science; that is, we have an all- 
pervading medium, an elastic solid, with a great degree of rigidity — 
a rigidity so prodigious in proportion to its density that the vibra- 
tions of light in it have the frequencies I have mentioned, with the 
wave-lengths I have mentioned. The fundamental question as to 
whether or not luminiferous ether has gravity has not been answered. 
We have no knowledge that the luminiferous ether is attracted by 
gravity; it is sometimes called imponderable because some people 
vainly imagine that it has no weight: I call it matter with the same 
kind of rigidity that this elastic jelly has. 

Here are two tourmalines; if you look through them toward the 
light you see the white light all round, i. e. they are transparent. If 
I turn round one of these tourmalines the light is extinguished, it is 
absolutely black, as though the tourmalines were opaque. This is an 
illustration of what is called polarisation of light. I cannot speak 
to you about qualities of light without speaking of the polarisation of 



THE WAVE THEORY OF LIGHT 265 

light. I want to show you a most beautiful eflect of polarising light, 
before illustrating a little further by means of this large mechanical 
illustration which you have in the bowl of jelly. What you saw first 
were two plates of the crystal tourmaline (which came from Brazil, 
I believe) having the property of letting light pass when both plates 
are placed in one particular direction as regards their axes of crystal- 
lisation, and extinguishing it when it passes through them with one 
of the plates held in another direction. Now I put in the lantern 
an instrument called a "Nicol prism," which also gives rays of 
polarised light. A Nicol prism is a piece of Iceland spar, cut in two 
and turned one part relatively to the other in a very ingenious way, 
and put together again and cemented into one by Canada balsam. 
The Nicol prism takes advantage of the property which the spar has 
of double refraction, and produces the phenomenon which I now 
show you. I turn one prism round in a certain direction and you 
get light — a maximum of light. I turn it through a right angle 
and you get blackness. I turn it one quarter round again, and get 
maximum light; one quarter more, maximum blackness; one quar- 
ter more, and bright light. We rarely have a grand specimen of a 
Nicol prism as this. 

There is another way of producing polarised light. I stand before 
that light and look at its reflection in a plate of glass on the table 
through one of the Nicol prisms, which I turn round, so. Now if I 
incline that plate of glass at a particular angle — rather more than 
fifty-five degrees — I find a particular position in which, if I look at it 
and then turn the prism round in the hand, the effect is absolutely 
to extinguish the light in one position of the prism and to give it 
maximum brightness in another position. I use the term "absolute" 
somewhat rashly. It is only a reduction to a very small quantity of 
light, not an absolute annulment as we have in the case of the two 
Nicol prisms used conjointly. As to the mechanics of the thing, those 
of you who have never heard of this before would not know what I 
was talking about; it could only be explained to you by a course of 
lectures in physical optics. The thing is this, vibrations of light must 
be in a definite direction relatively to the line in which the light 
travels. 

Look at this diagram, the light goes from left to right; we have 



266 KELVIN 

vibrations perpendicular to the line of transmission. There is a line 
up and down which is the line of vibration. Imagine here a source 
of light, violet light, and here in front of it is the line of propagation. 
Sound-vibrations are to and fro in, this is transverse to, the line of 
propagation. Here is another, perpendicular to the diagram, still 
following the law of transverse vibration; here is another, circular 
vibration. Imagine a long rope, you whirl one end of it and you see 
a screw-like motion running along, and you can get this circular 
motion in one direction or in the opposite. 

Plane-polarised light is light with the vibrations all in a single 
plane, perpendicular to the plane through the ray which is technically 
called the "plane of polarisation." Circularly polarised light consists 
of undulations of luminiferous ether having a circular motion. Ellip- 
tically polarised light is something between the two, not in a straight 
line, and not in a circular line; the course of vibration is an ellipse. 
Polarised light is light that performs its motions continually in one 
mode or direction. If in a straight line it is plane-polarised; if in a 
circular direction it is circularly polarised light; when elliptical it is 
elliptically polarised light. 

With Iceland spar, one unpolarised ray of light divides on entering 
it into two rays of polarised light, by reason of its power of double 
refraction, and the vibrations are perpendicular to one another in the 
two emerging rays. Light is always polarised when it is reflected 
from a plate of unsilvered glass, or from water, at a certain definite 
angle of fifty-six degrees for glass, fifty-two degrees for water, the 
angle being reckoned in each case from a perpendicular to the sur- 
face. The angle for water is the angle whose tangent is 1.4. I wish 
you to look at the polarisation with your own eyes. Light from glass 
at fifty-six degrees and from water at fifty-two degrees goes away 
vibrating perpendicularly to the plane of incidence and plane of re- 
flection. 

We can distinguish it without the aid of an instrument. There 
is a phenomenon well known in physical optics as "Haidinger's 
Brushes." The discoverer is well known in Philadelphia as a miner- 
alogist, and the phenomenon I speak of goes by his name. Look at 
the sky in a direction of ninety degrees from the sun, and you will 
see a yellow and blue cross, with the yellow toward the sun, and 



THE WAVE THEORY OF LIGHT 267 

from the sun, spreading out like two foxes' tails with blue between, 
and then two red brushes in the space at right angles to the blue. If 
you do not see it, it is because your eyes are not sensitive enough, 
but a little training will give them the needed sensitiveness. If you 
cannot see it in this way try another method. Look into a pail of 
water with a black bottom; or take a clear glass dish of water, rest it 
on a black cloth, and look down at the surface of the water on a day 
with a white cloudy sky (if there is such a thing ever to be seen in 
Philadelphia). You will see the white sky reflected in the basin of 
water at an angle of about fifty degrees. Look at it with the head 
tipped on one side and then again with the head tipped to the 
other side, keeping your eyes on the water, and you will see Haiding- 
er's brushes. Do not do it fast or you will make yourself giddy. The 
explanation of this is the refreshing of the sensibility of the retina. 
The Haidinger's brush is always there, but you do not see it because 
your eye is not sensitive enough. After once seeing it you always see 
it; it does not thrust itself inconveniently before you when you do 
not want to see it. You can also readily see it in a piece of glass with 
a dark cloth below it, or in a basin of water. 

I am going to conclude by telling you how we know the wave- 
lengths of light, and how we know the frequency of the vibrations, 
and we shall actually make a measurement of the wave-length of 
yellow light. I am now going to show you the diffraction spectrum. 

You see on the screen,^ on each side of a central white bar of light, 
a set of bars of light, of variegated colours, the first one on each side 
showing blue or indigo colour, about four inches from the central 
white bar, and red about four inches farther, with vivid green be- 
tween the blue and the red. That effect is produced by a grating with 
400 lines to the centimetre, engraved on glass, which I now hold in 
my hand. The next grating that we shall try has 3,000 lines on a 
Paris inch. You see the central space and on each side a large 
number of spectrums, blue at one end and red at the other. The 
fact that, in the first spectrum, red is about twice as far from the 
centre as the blue, proves that a wave-length of red light is double 
that of blue light. 

I will now show you the operation of measuring the length of a 

' Showing the chromatic bands thrown upon the screen from a diffraction grating. 



268 KELVIN 

wave of sodium light, that is a light like that marked D on the 
spectrum (Fig. 120), a light produced by a spirit-lamp with salt in it. 
The sodium vapour is heated up to several thousand degrees, when 
it becomes self-luminous and gives such a light as we get by throwing 
salt upon a spirit-lamp in the game of snap-dragon. 

I hold in my hand a beautiful grating of glass silvered by Liebig's 
process with metallic silver, a grating with 6,480 lines to the inch, 
belonging to my friend Professor Barker, which he has kindly 
brought here for us this evening. You will see the brilliancy of colour 
as I turn the light reflected from the grating toward you and pass 
the beam round the room. You have now seen directly with your 
own eyes these brilliant colours reflected from the grating, and you 
have also seen them thrown upon the screen from a grating placed 
in the lantern. Now with a grating of 17,000 lines per inch — a much 
greater number than the other — you will see how much further from 
the central bright space the first spectrum is; how much more this 
grating changes the direction, or diffraction, of the beam of light. 
Here is the centre of the grating, and there is the first spectrum. You 
v\dll note that the violet light is least diffracted and the red light is 
most diffracted. This diffraction of light first proved to us definitely 
the reality of the undulatory theory of light. 

You ask why does not light go round a corner as sound does. 
Light does go round a corner in these diffraction spectrums; and it 
is shown going round a corner, since it passes through these bars 
and is turned round an angle of thirty degrees. The phenomena of 
light going round a corner seen by means of instruments adapted 
to show the result and to measure the angles through which it is 
turned, is called the diffraction of light. 

I can show you an instrument which will measure the wave-lengths 
of light. Without proving the formula, let me tell it to you. A spirit- 
lamp with salt sprinkled on the wick gives very nearly homogeneous 
light, that is to say, light of one wave-length, or all of the same period. 
I have here a little grating which I take in my hand. I look through 
this grating and see that candle before me. Close behind it you see 
a blackened slip of wood with two white marks on it ten inches 
asunder. The line on which they are marked is placed perpendicular 
to the line at which I shall go from it. When I look at this salted 



THE WAVE THEORY OF LIGHT 269 

spirit-lamp I see a series o£ spectrums of yellow light. As I am some- 
what short-sighted I am making my eye see with this eye-glass and 
the natural lenses of the eye what a long-sighted person would make 
out without an eye-glass. On that screen you saw a succession of 
spectrums. I now look direct at the candle and what do I see? I see 
a succession of five or six brilliantly coloured spectrums on each side 
of the candle. But when I look at the salted spirit-lamp, now I see 
ten spectrums on one side and ten on the other, each of which is a 
monochromatic band of light. 

I will measure the wave-length of the light thus. I walk away to a 
considerable distance and look at the spirit-lamp and marks. I see a 
set of spectrums. The first white line is exactly behind the flame. I 
want the first spectrum to the right of that white line to fall exactly 
on the other white line, which is ten inches from the first. As I walk 
away from it I see it is now very near it; it is now on it. Now the 
distance from my eye is to be measured, and the problem is again to 
reduce feet to inches. The distance from the spectrum of the flame 
to my eye is thirty-four feet nine inches. Mr. President, how many 
inches is that? 417 inches, in round numbers 420 inches. Then we 
have the proportion, as 420 is to 10 so is the length from bar to bar 
of the grating to the wave-length of sodium light. That is to say as 
forty-two is to one. The distance from bar to bar is the four hun- 
dredth of a centimetre: therefore the forty-second part of the four 
hundredth of a centimetre is the wave-length according to our 
simple, and easy and hasty experiment. The true wave-length of 
sodium light, according to the most accurate measurement, is about 
a 17,000th of a centimetre, which differs by scarcely more than one 
per cent, from our result! 

The only apparatus you see is this little grating — a piece of glass 
having a space four-tenths of an inch wide ruled with 400 fine lines. 
Any of you who will take the trouble to buy one may measure the 
wave-length of a candle flame himself. I hope some of you will be 
induced to make the experiment for yourselves. 

If I put salt on the flame of a spirit-lamp, what do I see through 
this grating? I see merely a sharply defined yellow light, constituting 
the spectrum of vaporised sodium, while from the candle flame I see 
an exquisitely coloured spectrum, far more beautiful than that I 



270 KELVIN 

showed you on the screen. I see in fact a series of spectrums on the 
two sides with the blue toward the candle flame and the red further 
out. I cannot get one definite thing to measure from in the spec- 
trum from the candle flame, as I can with the flame of a spirit-lamp 
with the salt thrown on it, which gives as I have said a simple yellow 
light. The highest blue light I see in the candle flame is now exactly 
on the line. Now measure to my eye, it is forty-four feet four inches, 
or 532 inches. The length of this wave then is the 532d part of the 
four hundredth of a centimetre which would be the 21,280th of a 
centimetre, say the 21,000th of a centimetre. Then measure for the 
red and you will find something like the ii,oooth for the lowest of 
the red light. 
Lastly, how do we know the frequency of vibration.' 
Why, by the velocity of light. How do we know that ? We know 
it in a number of different ways, which I cannot explain now because 
time forbids, and I can now only tell you shortly that the frequency of 
vibration for any particular ray is equal to the velocity of light 
divided by the wave-length for that ray. The velocity of light is 
about 187,000 British statute miles per second, but it is much better 
to take the kilometre — which is about six-tenths of a mile— for the 
unit, when we find the velocity is very accurately 300,000 kilometres, 
or 30,000,000,000 centimetres, per second. Take now the wave-length 
of sodium light, as we have j ust measured it by means of the salted 
spirit-lamp, to be one 17,000th of a centimetre, and we find the fre- 
quency of vibration of the sodium light to be 510 million million per 
second. There, then, you have a calculation of the frequency from a 
simple observation which you all can make for yourselves. 

Lastly, I must tell you about the colour of the blue sky which is 
illustrated by this spherule imbedded in an elastic solid (Fig. 121). 
I want to explain to you in two minutes the mode of vibration. Take 
the simplest plane-polarised light. Here is a spherule which is pro- 
ducing it in an elastic solid. Imagine the solid to extend miles 
horizontally and miles up and down, and imagine this spherule to 
vibrate up and down. It is quite clear that it will make transverse 
vibrations similarly in all horizontal directions. The plane of polari- 
sation is defined as a plane perpendicular to the line of vibration. 
Thus, light produced by a molecule vibrating up and down, as this 



THE WAVE THEORY OF LIGHT 27I 

red globe in the jelly before you, is polarised in a horizontal plane 
because the vibrations are vertical. 

Here is another mode of vibration. Let me twist this spherule in 
the jelly as I am now doing, and that will produce vibrations, also 
spreading out equally in all horizontal directions. When I twist this 
globe round it draws the jelly round with it; twist it rapidly back 
and the jelly flies back. By the inertia of the jelly the vibrations 
spread in all directions and the lines of vibration are horizontal all 
through the jelly. Everywhere, miles away that solid is placed in 



// f \ \ 




\ 

Fio. 121 — Vibrating Spherule Imbedded in an Elastic Solid 

vibration. You do not see the vibrations, but you must understand 
that they are there. If it flies back it makes vibration, and we have 
waves of horizontal vibrations travelling out in all directions from 
the exciting molecule. 

I am now causing the red globe to vibrate to and fro horizontally. 
That will cause vibrations to be produced which will be parallel to 
the line of motion at all places of the plane perpendicular to the 
range of the exciting molecule. What makes the blue sky? These 
are exactly the motions that make the blue light of the sky, which is 
due to spherules in the luminiferous ether, but little modified by the 
air. Think of the sun near the horizon, think of the light of the sun 
streaming through and giving you the azure blue and violet over- 
head. Think first of any one particle and think of it moving in such 
a way as to give horizontal and vertical vibrations and circular and 
elliptic vibrations. 

You see the blue sky in high pressure steam blown into the air; 
you see it in the experiment of Tyndall's blue sky in which a delicate 
condensation of vapour gives rise to exactly the azure blue of the sky. 

Now the motion of the luminiferous ether relatively to the spherule 
gives rise to the same effect as would an opposite motion impressed 
upon the spherule quite independently by an independent force. So 



272 KELVIN 

you may think of the blue colour coming from the sky as being 
produced by to and fro vibrations of matter in the air, which vibrates 
much as this little globe vibrates imbedded in the jelly. 

The result in a general way is this: The light coming from the 
blue sky is polarised in a plane through the sun, but the blue light 
of the sky is complicated by a great number of circumstances and 
one of them is this, that the air is illuminated not only by the sun but 
by the earth. If we could get the earth covered by a black cloth 
then we could study the polarised light of the sky with a simplicity 
which we cannot do now. There are, in nature, reflections from the 
seas and rocks and hills and waters in an infinitely complicated man- 
ner. 

Let observers observe the blue sky not only in winter when the 
earth is covered with snow, but in summer when it is covered with 
dark green foliage. This will help to unravel the complicated phe- 
nomena in question. But the azure blue of the sky is light produced 
by the reaction on the vibrating ether of little spherules of water, of 
perhaps a fifty thousandth or a hundred thousandth of a centimetre 
diameter, or perhaps litde motes, or lumps, or crystals of common 
salt, or particles of dust, or germs of vegetable or animal species 
wafted about in the air. Now what is the luminiferous ether? It is 
matter prodigiously less dense than air — millions and millions and 
millions of times less dense than air. We can form some sort of idea 
of its limitations. We believe it is a real thing, with great rigidity in 
comparison with its density: it may be made to vibrate 400 million 
miUion times per second; and yet be of such density as not to produce 
the slightest resistance to any body going through it. 

Going back to the illustration of the shoemakers' wax; if a cork 
will, in the course of a year, push its way up through a plate of 
that wax when placed under water, and if a lead bullet will penetrate 
downwards to the bottom, what is the law of the resistance? It 
clearly depends on time. The cork slowly in the course of a year 
works its way up through two inches of that substance; give it one or 
two thousand years to do it and the resistance will be enormously 
less; thus the motion of a cork or bullet, at the rate of one inch in 
2,000 years, may be compared with that of the earth, moving at the 
rate of six times ninety-three million miles a year, or nineteen miles 



THE WAVE THEORY OF LIGHT 273 

per second, through the luminiferous ether; but when we can have 
actually before us a thing elastic like jelly and yielding like pitch, 
surely we have a large and solid ground for our faith in the specu- 
lative hypothesis of an elastic luminiferous ether, which constitutes 
the wave theory of light. 



THE TIDES 

[EVENING LECTURE TO THE BRITISH ASSOCIATION 

AT THE SOUTHAMPTON MEETING, FRIDAY, 

AUGUST 25, 1882] 

THE subject on which I have to speak this evening is the tides, 
and at the outset I feel in a curiously difficult position. If 
I were asked to tell what I mean by the Tides I should feel 
it exceedingly difficult to answer the question. The tides have some- 
thing to do with motion of the sea. Rise and fall of the sea is some- 
times called a tide; but I see, in the Admiralty Chart of the Firth 
of Clyde, the whole space between Ailsa Craig and the Ayrshire 
coast marked "very little tide here." Now, we find there a good ten 
feet rise and fall, and yet we are authoritatively told there is very little 
tide. The truth is, the word "tide" as used by sailors at sea means 
horizontal motion of the water; but when used by landsmen or sail- 
ors in port, it means vertical motion of the water. I hope my friend 
Sir Frederick Evans will allow me to say that we must take the 
designation in the chart, to which I have referred, as limited to the 
instruction of sailors navigating that part of the sea, and to say that 
there is a very considerable landsman's tide there — a rise and fall 
of the surface of the water relatively to the land — though there is 
exceedingly little current. 

One of the most interesting points of tidal theory is the determina- 
tion of the currents by which the rise and fall is produced, and so far 
the sailor's idea of what is most noteworthy as to tidal motion is 
correct: because before there can be a rise and fall of the water 
anywhere it must come from some other place, and the water cannot 
pass from place to place without moving horizontally, or nearly 
horizontally, through a great distance. Thus the primary phenom- 
enon of the tides is after all the tidal current; and it is the tidal 
currents that are referred to on charts where we have arrow-heads 

274 



THE TIDES 275 

marked with the statement that we have "very httle tide here," or 
that we have "strong tides" there. 

One instance of great interest is near Portland. We hear of the 
"race of Portland" which is produced by an exceedingly strong tidal 
current; but in Portland harbour there is exceedingly little rise and 
fall, and that little is much confused, as if the water did not know 
which way it was going to move. Sometimes the water rises, sinks, 
seems to think a litde while about it, and then rises again. The rise 
of the tide at Portland is interesting to the inhabitants of Southamp- 
ton in this, that whereas here, at Southampton, there is a double high 
water, there, at Portland, there is a double low water. The double 
high water seems to extend across the Channel. At Havre, and on 
the bar off the entrance to Havre, there is a double high water very 
useful to navigation; but Southampton I believe is pre-eminent 
above all the ports in the British Islands with respect to this con- 
venience. There is here (at Southampton) a good three hours of 
high water; — a little dip after the first high water, and then the water 
rises again a very little more for an hour and a half or two hours, 
before it begins to fall to low water. 

I shall endeavour to refer to this subject again. It is not merely the 
Isle of Wight that gives rise to the phenomenon. The influence ex- 
tends to the east as far as Christchurch, and is reversed at Portland, 
and we have the double or the prolonged high water also over at 
Havre; therefore, it is clearly not, as it has been supposed to be, due 
to the Isle of Wight. 

But now I must come back to the question, What are the "Tides" ? 
Is a "tidal wave" a tide? What is called in the newspapers a "tidal 
wave" rises sometimes in a few minutes, does great destruction, and 
goes down again, somewhat less rapidly. There are frequent in- 
stances in all parts of the world of the occurrence of that phenom- 
enon. Such motions of the water, however, are not tides; they are 
usually caused by earthquakes. But we are apt to call any not very 
short-time rise and fall of the water a tide, as when standing on the 
coast of a slandng shore where there are long ocean waves, we see the 
gradual sinkings and risings produced by them, and say that it is a 
wave we see, not a tide, till one comes which is exceptionally slow, 
and then we say "that is liker a tide than a wave." The fact is, 



276 KELVIN 

there is something perfectly continuous in the species of motion 
called wave, from the smallest ripple in a musical glass, whose 
period may be a thousandth of a second, to a "lop of water" in the 
Solent, whose period is one or two seconds, and thence on to the great 
ocean wave with a period of from fifteen to twenty seconds, where 
end the phenomenon which we commonly call waves (Fig. 122), and 



Fig. 122 — Wave forms 

not tides. But any rise and fall which is manifestly of longer period, 
or slower in its rise from lowest to highest, than a wind wave, we are 
apt to call a tide; and some of the phenomena that are analysed for, 
and worked out in this very tidal analysis that I am going to explain, 
are in point of fact more properly wind waves than true tides. 

Leaving these complicated questions, however, I will make a short 
cut, and assuming the cause without proving it, define the thing by 
the cause. I shall therefore define tides thus: Tides are motions of 
water on the earth, due to the attractions of the sun and of the moon. 
I cannot say tides are motions due to the actions of the sun and of the 
moon; for so I would include, under the designation of tide, every 
ripple that stirs a puddle or a millpond, and waves in the Solent or 
in the English Channel, and the long Atlantic wind waves, and the 
great swell of the ocean from one hemisphere to the other and back 
again (under the name which I find in the harmonic reduction of 
tidal observations), proved to take place once a year, and which I can 
only explain as the result of the sun's heat. 

But while the action of the sun's heat by means of the wind pro- 
duces ripples and waves of every size, it also produces a heaping-up 
of the water as illustrated by this diagram (Fig. 123). Suppose we 
have wind blowing across one side of a sheet of water, the wind 
ruffles the surface, the waves break if the wind is strong, and the 



THE TIDES 277 

result is a strong tangential force exerted by the wind on the surface 
water. If a ship is sailing over the water there is strong tangential 
force; thus the water is found going fast to leeward for a long dis- 
tance astern of a great ship sailing with a side wind: and, just as 
the sails of a ship standing high above the sea give a large area for 
the wind to act upon, every wave standing up gives a surface, and 
we have horizontal tangential force over the whole surface of a 
troubled sea. The result is that water is dragged along the surface 
from one side of the ocean to the other — from one side of the Atlantic 
to the other — and is heaped up on the side towards which the wind 
is blowing. To understand the dynamics of this phenomenon, think 
of a long straight canal with the wind blowing lengthwise along it. 
In virtue of the tangential force exerted on the surface of the water 
by the wind, and which increases with the speed of the wind, the 
water will become heaped up at one end of the canal, as shown in 
the diagram (Fig. 123), while the surface water throughout the 



Fig. 123 — Showing the heaping-up o£ water produced by wind 

whole length will be observed moving in the direction of the wind — 
say in the direction of the two arrows near to the surface of the water 
above and below it. But to re-establish the disturbed hydrostatic 
equilibrium, the water so heaped up will tend to flow back to the 
end from which it has been displaced, and as the wind prevents this 
taking place by a surface current, there will be set up a return cur- 
rent along the bottom of the canal, in a direction opposite to that of 
the wind, as indicated by the lowermost arrow in the diagram (Fig. 
123). The return current in the ocean, however, is not always an 
under current, such as I have indicated in the diagram, but may 
sometimes be a lateral current. Thus a gale of wind blowing over 
ten degrees of latitude will cause a drag of water at the surface, 
but the return current may be not an under current but a current on 



278 KELVIN 

one side or the other of the area affected by the wind. Suppose, for 
instance, in the Mediterranean there is a strong east wind blowing 
along the African coast, the result will be a current from east to west 
along that coast, and return current along the northern coasts of 
the Mediterranean. 

The rise and fall of the water due to these motions are almost in- 
extricably mixed up with the true tidal rise and fall. 

There is another rise and fall, also connected with the heating 
effect of the sun, that I do not call a true tide, and that is a rise and 
fall due to change of atmospheric pressure. When the barometer is 
high over a large area of ocean, then, there and in neighboring places, 
the tendency to hydrostatic equilibrium causes the surface of the 
water to be lower, where it is pushed down by the greater weight of 
air, and to be higher where there is less weight over it. It does not 
follow that in every case it is lower, because there may not be time 
to produce the effect, but there is this tendency. It is very well known 
that two or three days of low barometer make higher tides on our 
coast. In Scotland and England and Ireland, two or three days of 
low barometer generally produce all round the shore higher water 
than when the barometer is high; and this effect is chiefly noticed 
at the time of tidal high water, because people take less notice of 
low water — as at Portland where they think nothing of the double 
low water. Hence we hear continually of very high tides — very 
high water noticed at the time of high tides — when the barometer is 
low. We have not always, however, in this effect of barometric 
pressure really great tidal rise and fall. On the contrary we have the 
curious phenomenon that sometimes when the barometer is very 
low, and there are gales in the neighbourhood, there is very little 
rise and jail, as the water is \ept heaped up and does not sink by 
anything like its usual amount from the extra high level that it has 
at high water. But I fear I have got into questions which are leading 
me away from my subject, and as I cannot get through them I must 
just turn back. 

Now think of the definition which I gave of the "tides," and 
think of the sun alone. The action of the sun cannot be defined as 
the cause of the solar tides. Solar tides are due to action of the sun, 
but all risings and fallings of the water due to the action of the sun 



THE TIDES 279 

are not tides. We want the quantification of the predicate here 
very badly. We have a true tide depending on the sun, the mean solar 
diurnal tide, having for its period twenty-four solar hours, which is 
inextricably mixed up with those meteorological tides that I have 
just been speaking of — tides depending on the sun's heat, and on the 
variation of the direction of the wind, and on the variation of baro- 
metric pressure according to the time of day. The consequence is 
that in tidal analysis, when we come to the solar tides, we cannot 
know how much of the analysed result is due to attraction, and how 
much to heating effect directly or indirectly, whether on water, or 
on air, or on water as affected by air. As to the lunar tides we are 
quite sure of them; — they are gravitational, and nothing but gravita- 
tional; but I hope to speak later of the supposed relation of the moon 
to the weather, and the relation that has to the tides. 

I have defined the tides as motions of water on the earth due to 
the attractions of the sun and of the moon. How are we to find out 
whether an observed motion of the water is a tide or is not a tide as 
thus defined? Only by the combination of theory and observation: if 
these afford sufficient reason for believing that the motion is due to 
attraction of the sun or of the moon, or of both, then we must call 
it a tide. 

It is curious to look back on the knowledge of the tides possessed 
in ancient times, and to find as early as two hundred years before 
the Christian era a very clear account given of the tides at Cadiz. 
But the Romans generally, knowing only the Mediterranean, had 
not much clear knowledge of the tides. At a much later time than 
that, we hear from the ancient Greek writers and explorers — Posi- 
donius, Strabo, and others — that in certain remote parts of the 
world, in Thule, in Britain, in Gaul, and on the distant coasts of 
Spain, there were motions of the sea — a rising and falling of the 
water — which depended in some way on the moon. Julius Caesar 
came to know something about it; but it is certain the Roman 
Admiralty did not supply Julius Csesar's captains with tide tables 
when he sailed from the Mediterranean with his expeditionary 
force, destined to put down anarchy in Britain. He says, referring 
to the fourth day after his first landing in Britain — "That night it 
happened to be full moon, which time is accustomed to give the 



28o KELVIN 

greatest risings o£ water in the ocean, though our people did not 
know it." It has been supposed however that some of his people did 
know it — some of his quartermasters had been in England before 
and did know — ^but that the discipline in the Roman navy was so 
good that they had no right to obtrude their knowledge; and so, 
although a storm was raging at the time, he was not told that the 
water would rise in the night higher than usual, and nothing was 
done to make his transports secure higher up on the shore while he 
was fighting the Britons. After the accident Caesar was no doubt 
told — "Oh, we knew that before, but it might have been ill taken 
if we had said so." 

Strabo says — "Soon after moonrise the sea begins to swell up and 
flow over the earth till the moon reaches mid heaven. As she de- 
scends thence the sea recedes till about moonset, when the water is 
lowest. It then rises again as the moon, below the horizon, sinks 
to mid heaven under the earth." It is interesting here to find the 
tides described simply with reference to the moon. But there is some- 
thing more in this ancient account of Strabo; he says, quoting Posi- 
donius — "This is the daily circuit of the sea. Moreover, there is a 
regular monthly course, according to which the greatest rise and 
fall takes place about new moon, then diminishing rise and fall till 
half moon, and again increasing till full moon." And lastly he refers 
to a hearsay report of the Gaditani (Cadizians) regarding an annual 
period in the amount of the daily rise and fall of the sea, which 
seems to be not altogether right, and is confessedly in part conjectural. 
He gave no theory, of course, and he avoided the complication of 
referring to the sun. But the mere mention of an annual period is 
interesting in the history of tidal theory, as suggesting that the 
rises and falls are due not to the moon alone but to the sun also. 
The account given by Posidonius is fairly descriptive of what occurs 
at the present day at Cadiz. Exactly the opposite would be true at 
many places; but at Cadiz the time of high water at new and full 
moon is nearly twelve o'clock. Still, I say we have only definition 
to keep us clear of ambiguities and errors; and yet, to say that those 
motions of the sea which we call tides depend on the moon, was con- 
sidered, even by Galileo, to be a lamentable piece of mysticism which 
he read with regret in the writings of so renowned an author as 
Kepler. 



THE TIDES 281 

It is indeed impossible to avoid theorising. The first who gave a 
theory was Newton; and I shall now attempt to speak of it suf- 
ficiently to allow us to have it as a foundation for estimating the 
forces with which we are concerned, in dealing with some of the 
very perplexing questions which tidal phenomena present. 

We are to imagine the moon as attracting the earth, subject to the 
forces that the different bodies exert upon each other. We are not 
to take Hegel's theory — that the Earth and the Planets do not move 
like stones, but move along like blessed gods, each an independent 
being. If Hegel had any grain of philosophy in his ideas of the solar 
system, Newton is all wrong in his theory of the tides. Newton con- 
sidered the attraction of the sun upon the earth and the moon, of 
the earth upon the moon, and the mutual attractions of different parts 
of the earth; and left it for Cavendish to complete the discovery of 
gravitation, by exhibiting the mutual attraction of two pieces of 
lead in his balance. Tidal theory is one strong link in the grand 
philosophic chain of the Newtonian theory of gravitation. In ex- 
plaining the tide-generating force we are brought face to face with 
some of the subtleties, and with some of the mere elements, of 
physical astronomy. I will not enter into details, as it would be 
useless for those who already understand the tidal theory, and un- 
intelligible to those who do not. 

I may just say that the moon attracts a piece of matter, for example 
a pound-weight, here on the earth, with a force which we compare 
with the earth's attraction thus. Her mass is ^ of the earth's, and 
she is sixty times as far away from the earth's centre as we are 
here. Newton's theory of gravitation shows, that when you get 
outside the mass of the earth the resultant attraction of the earth on 
the pound weight, is the same as if the whole mass of the earth were 
collected at the centre, and that it varies inversely as the square of 
the distance from the centre. The same law is inferred regarding 
the moon's attraction from the general theory. The moon's attraction 

on this pound weight is therefore _I5^ , or ags^.ooo of the attraction 

'^ ° 60X60 

of the earth on the same mass. But that is not the tide-generating 
force. The moon attracts any mass at the nearest parts of the earth's 
surface with greater force than an equal mass near the centre; and 
attracts a mass belonging to the remoter parts with less force. 



282 KELVIN 

Imagine a point where the moon is overhead, and imagine another 
point on the surface of the earth at the other end of a diameter 
passing through the first point and the centre of the earth (illustrated 
by B and A of Fig. 124, p. 284). The moon attracts the nearest 
point (B) with a force which is greater than that with which it 
attracts the farther point (A) in the ratio of the square of 59 to 
the square of 61. Hence the moon's attraction on equal masses at 
the nearest and farthest points differs by one fifteenth part of her 
attraction on an equal mass at the earth's centre, or about a 4,320,- 
oooth, or, roughly, a four-millionth, of the earth's attraction on an 
equal mass at its surface. Consequently the water tends to protrude 
towards the moon and from the moon. If the moon and earth were 
held together by a rigid bar the water would be drawn to the side 
nearest to the moon— drawn to a prodigious height of several hun- 
dred feet. But the earth and moon are not so connected. We may 
imagine the earth as falling towards the moon, and the moon as fall- 
ing towards the earth, but never coming nearer; the bodies, in reality, 
revolving round their common centre of gravity. A point nearest to 
the moon is as it were dragged away from the earth, and thus the 
result is that apparent gravity differs by about one four-millionth 
at the points nearest to and farthest from the moon. At the inter- 
mediate points of the circle C, D (Fig. 124, p. 284), there is a some- 
what complicated action according to which gravitation is increased 
by about one 17-millionth, and its direction altered by about one 17- 
millionth, so that a pendulum 17,000 feet long, a plummet rather 
longer than from the top of Mont Blanc to sea level, would, if show- 
ing truly the lunar disturbing force, be deflected through a space of 
one thousandth of a foot. It seems quite hopeless by a plummet to 
exhibit the lunar disturbance of gravity. A spring balance to show 
the alteration of magnitude, and a plummet to show the change of 
direction are conceivable; but we can scarcely believe that either 
can ever be produced, with sufficient delicacy and consistency and 
accuracy to indicate these results. 

A most earnest and persevering effort has been made by Mr. 
George Darwin and Mr. Horace Darwin to detect variations in grav- 
ity due to lunar disturbance, and they have made apparatus which, 
notwithstanding the prodigious smallness of the effect to be observed, 



THE TIDES 283 

is in point of delicacy and consistency capable of showing it; but 
when they had got their delicate pendulum— their delicate plummet 
about the length of an ordinary seconds' pendulum — and their deli- 
cate multiplying gear to multiply the motion of its lower end by 
about a million times, and to show the result on a scale by the reflec- 
tion of a ray of light, they found the little image incessantly moving 
backward and forward on the scale with no consistency or regu- 
larity; and they have come to the conclusion that there are continual 
local variations of apparent gravity taking place for which we know 
no rule, and which are considerably greater than the lunar disturb- 
ance for which they were seeking. That which they found — continual 
motions of the surface of the earth, and which was not the primary 
object of their investigation — is in some respects more interesting than 
what they sought and did not find. The delicate investigation thus 
opened up promises a rich harvest of knowledge. These disturbances 
are connected with earthquakes such as have been observed in a 
very scientific and accurate manner by Milne, Thomas Gray, and 
Ewing in Japan, and in Italy by many accurate observers. All such 
observations agree in showing continual tremor and palpitation of 
the earth in every part. 

One other phenomenon that I may just refer to now as coming 
out from tide-gauge observations, is a phenomenon called seiches by 
Forel, and described by him as having been observed in the lakes 
of Geneva and Constance. He attributes them to differences of 
barometric pressure at the ends of the lake, and it is probable that 
part of the phenomenon is due to such differences. I think it is 
certain, however, that the whole is not due to such differences. The 
Portland tide curve and those of many other places, notably the tide 
curve for Malta, taken about ten years ago by Sir Cooper Key, and 
observations on the Atlantic coasts and in many other parts of the 
world, show something of these phenomena; a ripple or roughness 
on the curve traced by the tide gauge, which, when carefully looked 
to, indicates a variation not regular but in some such period as 
twenty or twenty-five minutes. It has been suggested that they are 
caused by electric action! Whenever the cause of a thing is not 
known it is immediately put down as electrical! 

I would like to explain to you the equilibrium theory, and the 



284 KELVIN 

kinetic theory, of the tides, but I am afraid I must merely say there 
are such things; and that Laplace in his great work, his Mecanique 
Celeste, first showed that the equilibrium theory was utterly insuffi- 
cient to account for the phenomena, and gave the true principles of 
the dynamic action on which they depend. The resultant effect of 
the tide-generating force is to cause the water to tend to become 
protuberant towards the moon and the sun and from them, when 
they are in the same straight line, and to take a regular spheroidal 





Fig. 124 — Spring Tides 




Fig. 125 — Spring Tides 

form, in which the difference between the greatest and the least 
semi-diameter is about 2 feet for lunar action alone, and i foot for 
the action of the sun alone — that is a tide which amounts to 3 feet 
when the sun and moon act together (Figs. 124 and 125), and to i 
foot only when they act at cross purposes (Figs. 126 and 127), so 
as to produce opposite effects. These diagrams. Figs. 124 to 127, illus- 
trate spring and neap tides: the dark shading around the globe, E, 
representing a water envelope surrounding the earth. There has 
been much discussion on the origin of the word neap. It seems to be 
an Anglo-Saxon word meaning scanty. Spring seems to be the same 
as when we speak of plants springing up. I well remember at the 
meeting of the British Association at Edinburgh a French mem- 
ber who, meaning spring tides, spoke of the grandes marees du prin- 
temps. Now you laugh at this; and yet, though he did not mean it, 
he was quite right, for the spring tides in the spring time are greater 



THE TIDES 



285 




Fig. 126 — ^Neap Tides 





Fig. 127 — ^Neap Tides 




286 KELVIN 

on the whole than those at other times, and we have the greatest 
spring tides in the spring of the year. But there the analogy ceases, 
for we have also very high spring tides in autumn. Still the meaning 
of the two words is the same etymologically. Neap tides are scanty 
tides, and spring tides are tides which spring up to remarkably 
great heights. 

The equilibrium theory of the tides is a way of putting tidal 
phenomena. We say the tides would be so and so if the water took 
the figure of equilibrium. Now the water does not cover the whole 
earth, as we have assumed in the diagrams (Figs. 124-127), but 
the surface of the water may be imagined as taking the same figure, 
so far as there is water, that it would take if there were water over 
the whole surface of the earth. But here a difficult question comes 
in — namely, the attraction of the water for parts of itself. If we 
consider the water flowing over the whole earth this attraction must 
be taken into account. If we imagine the water of exceedingly small 
density so that its attraction on itself is insensible compared with 
that of the earth, we have thus to think of the equilibrium theory. 
But, on the other hand, if the water had the same density as the 
earth, the result would be that the solid nucleus would be almost 
ready to float; and now imagine that the water is denser than the 
earth, and we put the tides out of consideration altogether. Think 
of the earth covered over with mercury instead of water — a layer of 
mercury a foot deep. The solid earth would tend to float, and would 
float, and the result would be that the denser liquid would run to, 
and cover one side up to a certain depth, and the earth would be 
as it were floating out of the sea. That explains one curious result 
that Laplace seems to have been much struck with: the stability 
of the ocean requires that the density of the water should be less 
than that of the solid earth. But take the sea as having the specific 
gravity of water, the mean density of the earth is only 5.6 times that 
of water, and this is not enough to prevent the attraction of water 
for water from being sensible. Owing therefore to the attraction of 
the water for parts of itself the tidal phenomena are somewhat 
larger than they would be without it, but neglecting this, and 
neglecting the deformation of the solid earth, we have the ordinary 
equilibrium theory. 



THE TIDES 287 

Why does the water not follow the equilibrium theory? Why 
have we tides of 20 feet or 30 feet or 40 feet in some places, and 
only of 2 or 3 feet in others ? Because the water has not time in the 
course of 12 hours to take the equilibrium figure, and because after 
tending towards it, the water runs beyond it. 

I ask you to think of the oscillations of water in a trough. Look at 
this diagram (Fig. 128), which will help you to understand how the 




Fig. 128 — Oscillations of Water in a Trough 

tidal efJect is prodigiously magnified by a dynamical action due to the 
inertia of the water. The tendency of water in motion to keep its 
motion prevents it from taking the figure of equilibrium. [A chart 
showing the tides of the English Channel was exhibited, from which 
it was seen that while at Dover there were tides of 21 feet, there was 
at Portland very little rise and fall.] Imagine a canal instead of the 
English Channel, a canal stopped at the Straits of Dover and at the 
opposite end at Land's End, and imagine somehow a disturbing force 
causing the water to be heaped up at one end. There would be a 
swing of water from one end to the other, and if the period of the 
disturbing force approximately agreed with the period of free oscil- 
lation, the effect would be that the rise and fall would go vastly 
above and below the range due to equilibrium action. Hence it is 
we have the 21 feet rise and fall at Dover. The very little rise and fall 
at Portland is also illustrated in the uppermost figure of this diagram 
(Fig. 128). Thus high water at Dover is low water at Land's End, 
and the water seesaws as it were about a line going across from Port- 
land to Havre (represented by N in the figure) ; not a line going 



288 KELVIN 

directly across, however, for on the other side of the Channel there 
is a curious complication. 

At the time of high water at Dover there is hardly any current 
in the Channel. As soon as the water begins to fall at Dover the 
current begins to flow west through the whole of the Channel. When 
it is mid-tide at Dover the tide is flowing fastest in the Channel. 
This was first brought to light by Admiral Beechey. 

I wish I had time to show the similar theory as to the tides in 
the Irish Channel. The water runs up the English Channel to 
Dover, and up the Irish Channel to fill up the basin round the Isle 
of Man. Take the northern mouth of the Irish Channel between the 
Mull of Cantire and the northeast coast of Ireland. The water 
rushes in through the straits between Cantire and Rathlin Island, 
to fill up the Bay of Liverpool and the great area of water round 
the Isle of Man. This tidal wave entering from the north, running 
southward through the Channel, meets in the Liverpool basin with 
the tidal stream coming from the south entrance, and causes the 
time of high water at Liverpool to be within half-an-hour of the time 
of no currents in the northern and southern parts of the Channel. 

I would like to read you the late Astronomer-Royal's appreciation 
of Laplace's splendid work on the tides. 

Airy says of Laplace: "If now, putting from our thoughts the 
details of the investigation, we consider its general plan and objects, 
we must allow it to be one of the most splendid works of the greatest 
mathematician of the past age. To appreciate this, the reader must 
consider, first, the boldness of the writer, who, having a clear under- 
standing of the gross imperfections in the methods of his predeces- 
sors, had also the courage deliberately to take up the problem on 
grounds fundamentally correct (however it might be limited by sup- 
positions afterwards introduced) ; secondly, the general difficulty of 
treating the motion of fluids; thirdly, the pecuHar difficulty of treat- 
ing the motions when the fluids cover an area which is not plane 
but convex; and fourthly, the sagacity of perceiving that it was 
necessary to consider the earth as a revolving body, and the skill of 
correctly introducing this consideration. This last point alone, in 
our opinion, gives a greater claim for reputation than the boasted 
explanation of the long inequaUty of Jupiter and Saturn." 



THE TIDES 289 

Tidal theory must be carried on along with tidal observations. In- 
struments for measuring and recording the height of the water at any 




Fig. 129 — ^Tide Gauge 

time give us results of observations.' Here is such an instrument — a 
tide gauge (Fig. 129) . The floater is made of thin sheet copper, and 

'The various instruments and tide-curves referred to in this lecture are fully 
described and illustrated in a paper on "The Tide Gauge, Tidal Harmonic Analyser, 
and Tide Predicter," read before the Institution of Civil Engineers, on ist March, 
1881, and published in their Proceedings for that date. 



290 KELVIN 

is suspended by a fine platinum wire. The vertical motion of the 
floater, as the water rises and falls, is transmitted, in a reduced pro- 
portion by a single pinion and wheel, to this frame or marker, which 
carries a small marking pencil. The paper on which the pencil 
marks the recording curve, is stretched on this cylinder, which, by 
means of the clockwork, is caused to make one revolution every 
twenty-four hours. The leaning-tower-of-Pisa arrangement of the 
paper-cylinder, and the extreme simplicity of the connection between 




Fig. 130 — Facsimile of weekly sheet of curves traced by Tide Gauge 

marker and floater, constitute the chief novelty. This tide gauge is 
similar to one now in actual use, recording the rise and fall of the 
water in the River Clyde, at the entrance to the Queen's Dock, 
Glasgow. A sheet bearing the curves (Fig. 130) traced by that ma- 
chine during a week is exhibited. 

After the observations have been taken, the next thing is to make 
use of them. Hitherto this has been done by laborious arithmetical 
calculation. I hold in my hand the Reports of the late Tidal Com- 
mittee of the British Association with the results of the harmonic 
analysis — about eight years' work carried on with great labour, and 
by aid of successive grants from the British Association. The Indian 
Government has continued the harmonic analysis for the seaports 
of India. The Tide Tables for Indian Ports for the Year 1882, issued 
under the authority of the Indian Government, show this analysis 
as in progress for the following ports, viz.: Aden, Kurrachee, Okha 
Point and Beyt Harbour at the entrance to the Gulf of Cutch, Bom- 
bay, Karwar, Beypore, Paumben Pass, Madras, Vizagapatam, Dia- 



THE TIDES 291 

mond Harbour, Fort Gloster and Kidderpore on the River Hooghly, 
Rangoon, Moulmein, and Port Blair. Mr. Roberts, who was first 
employed as calculator by the Committee of the British Association, 
has been asked to carry on the work for the Indian Government, 
and latterly, in India, native calculators under Major Baird, have 
worked by the methods and forms by which Mr. Roberts had worked 
in England for the British Association.^ The object is to find the 
values of the different tidal constituents. We want to separate out 
from the whole rise and fall of the ocean the part due to the sun, 




Fig. 131 — Declinational Tide 

the part due to the moon, the part due to one portion of the moon's 
effect, and the part due to another. There are complications depend- 
ing on the moon's position — declinational tides according as the 
moon is or is not in the plane of the earth's equator — and also on 
that of the sun. Thus we have the diurnal declinational tides. When 
the moon is in the north declination (Fig. 131) we have (in the 
equilibrium theory) higher water at lunar noon than at lunar mid- 
night. That difference in the height of high water, and the cor- 
responding solar noon tides and solar midnight tides, due to the sun 
not being in the earth's equator, constitute the lunar and solar diur- 
nal declinational tides. In summer the noon high water might be 

^ Note of September 17, 1887. On the subject of Tidal Harmonic Analysis see 
"Manual of Instructions for Tidal Observation," by Major Baird, published by Messrs. 
Taylor and Francis, London, 1886; also the Reports of the British Association Com- 
mittee "On Harmonic Analysis of Tidal Observations." — W. T. 



292 KELVIN 

expected to be higher than the mid- 
night high water, because the sun is 
nearer overhead to us than to our Ai> 
tipodes. 

By kind permission of Sir Fred- 
erick Evans, I am able to place before 
you these diagrams of curves drawn 
by Captain Harris, R. N., of the 
Hydrographic Department of the Ad- 
miralty, exhibiting the rise and fall of 
tides in Princess Royal Harbour, King 
George Sound, Western Australia, from 
January ist to December 31st, 1877, and 
in Broad Sound, Queensland, Australia, 
from July 15th, 1877, to July 23rd, 1878. 
Look at this one of these diagrams, a 
diagram of the tides at the northeast 
corner of Australia. For several days 
high water always at noon. When the 
tides are noticeable at all we have high 
water at noon, and when the tides are 
not at noon they are so small that they 
are not taken notice of at all. It thus 
appears as if the tides were irrespective 
of the moon, but they are not really so. 
When we look more closely, it is a full 
moon if we have a great tide at noon; or 
else it is new moon. It is at half moon 
that we have the small tides, and when 
they are smallest we have high water at 
six. There is also a great difference be- 
tween day and night high water; the 
difference between them is called the 
diurnal tide. A similar phenomenon 
is shown on a smaller scale in this 
curve, drawn by the first tide-predict- 



r 



THE TIDES 293 

ing machine. At a certain time the two high waters become equal- 
ised, and the two low waters very unequal (see p. 290 for real 
examples) . 

The object of the harmonic analysis is to analyse out from the com- 
plicated curve traced by the tide gauge the simplest harmonic ele- 
ments. A simple harmonic motion may be imagined as that of a 
body which moves simply up and down in a straight line, keep- 
ing level with the end of a clock hand, moving uniformly round. 
The exceedingly complicated motion that we have in the tides 
is analysed into a series of simple harmonic motions in different 
periods and with different amplitudes or ranges; and these sim- 
ple harmonic constituents added together give the complicated 
tides. 

All the work hitherto done has been accomplished by sheer cal- 
culation; but calculation of so methodical a kind that a machine 
ought to be found to do it. The Tidal Harmonic Analyser consists 
of an application of Professor James Thomson's disk-globe-and- 
cylinder integrator to the evaluation of the integrals required for 
the harmonic analysis. The principle of the machine and the essen- 
tial details are fully described and explained in papers communicated 
by Professor James Thomson and the author to the Royal Society, in 
1876 and 1878, and published in the Proceedings for those years;' 
also reprinted, with a postscript dated April, 1879, in Thomson and 
Tait's Natural Philosophy, Second Edition, Appendix B. It remains 
now to describe and explain the actual machine referred to in the 
last of these communications, which is the only tidal harmonic an- 
alyser hitherto made. It may be mentioned, however, in passing, that 
a similar instrument, with the simpler construction wanted for the 
simpler harmonic analysis of ordinary meteorological phenomena, 
has been constructed for the Meteorological Committee, and is now 
regularly at work at their office, harmonically analysing the results 
of meteorological observations, under the superintendence of Mr. 
R. H. Scott. 

Fig. 132 represents the tidal harmonic analyser, constructed under 
the author's direction, with the assistance of a grant from the Gov- 

' Vide vol. xxiv. p. 262, and vol. xxvii. p. 371. 



294 KELVIN 

ernment Grant Fund of the Royal Society. The eleven cranks of 

this instrument are allotted as follows: — 







Dis- 








tm- 




Cranks. 


Object. 


guish- 

ing 
Letter. 


Speed. 


I and 2 


To find the mean lunar semi-diurnal tide . . . 


M 


2 {y—a) 


3 .. 4 


„ mean solar „ „ . . . 


S 


2 (7—,) 


5 ,. 6 


„ luni-solar declinational diurnal tide 


K, 


y 


7 ., 8 


„ slower lunar „ „ 





(7—2.7) 


9 .. lo 


„ slower solar „ „ 


P 


(7—27;) 


II 


„ mean water level 


A, 





The general arrangement of the several parts may be seen from 
Fig. 132. The large circle at the back, near the center, is merely a 
counter to count the days, months, and years for four years, being the 
leap year period. It is driven by a worm carried on an intermediate 
shaft, with a toothed wheel geared on another on the solar shaft. 
In front of the centre is the paper drum, which is on the solar shaft, 
and goes round in the period corresponding to twelve mean solar 
hours. On the extreme left, the first pair of disks, with globes and 
cylinders, and crank shafts with cranks at right angles between them, 
driving their two cross-heads, corresponds to the Ki, or luni-solar 
diurnal tide. The next pair of disk-globe-and-cylinders corresponds 
to M, or the mean lunar semi-diurnal tide, the chief of all the tides. 
The next pair lie on the two sides of the main shaft carrying the 
paper drum, and correspond to S, the mean solar semi-diurnal tide. 
The first pair on the right correspond to O, or the lunar diurnal tide. 
The second pair on the right correspond to P, the solar diurnal tide. 
The last disk on the extreme right is simply Professor James Thom- 
son's disk-globe-and-cylinder integrator, applied to measure the area 
of the curve as it passes through the machine. 

The idle shafts for the M and the O tides are seen in front 
respectively on the left and right of the centre. The two other longer 
idle shafts for the K and the P tides are behind, and therefore not 
seen. That for the P tide serves also for the simple integrator on the 
extreme right. 

The large hollow square brass bar, stretching from end to end 



THE TIDES 295 

along the top of the instrument, and carrying the eleven forks rigidly 
attached to it, projecting downwards, is moved to and fro through 
the requisite range by a rack and pinion, worked by a handle and 
crank in front above the paper cylinder, a little to the right of its 
centre. Each of these eleven forks moves one of the eleven globes 
of the eleven disk-globe-and-cylinder integrators of which the ma- 
chine is composed. The other handle and crank in front, lower 
down and a little to the left of the centre, drives by a worm, at a con- 
veniently slow speed, the solar shaft, and through it, and the four 
idle shafts, the four other tidal shafts. 

To work the machine the operator turns with his left hand the 
driving crank, and with his right hand the tracing crank, by which 
the fork-bar is moved. His left hand he turns always in one direc- 
tion, and at as nearly constant a speed as is convenient to allow his 
right hand, alternately in contrary directions, to trace exactly with 
the steel pointer the tidal curve on the paper, which is carried across 
the line of to-and-fro motion of the pointer by the revolution of the 
paper drum, of which the speed is in simple proportion to the speed 
of the operator's left hand. 

The eleven little counters of the cylinders in front of the disks 
are to be set each at zero at the commencement of an operation, and 
to be read off from time to time during the operation, so as to give 
the value of the eleven integrals for as many particular values of the 
time as it is desired to have them. 

A first working model harmonic analyser, which served for 
model and for the meteorological analyser, now at work in the 
Meteorological Office, is here before you. It has five disk-globe-and- 
cylinders, and shafting geared for the ratio i : 2. Thus it serves to 
determine, from the deviation curve, the celebrated "A B C D E" of 
the Admiralty Compass Manual, this is to say, the coefficients in the 
harmonic expression 

A + B sin + C cosfl + D sin 20 + E cos 2B. 

for the deviation of the compass in an iron ship. 

The first instrument which I designed and constructed for use as 
a Tide Predictor was described in the Catalogue of the Loan Col- 
lection of Scientific Apparatus at South Kensington in 1876; and the 



296 KELVIN 

instrument itself was presented by the British Association to the 
South Kensington Museum, where it now is. The second instrument 
constructed on the same principle is in London, and is being worked 
under the direction of Mr. Roberts, analysing the tides for the Indian 
ports. The result of this work is these books {Tide Tables for In- 




FiG. 133 — ^Tide Predictor 

dian Ports) in which we have, for the first time, tables of the times 
and heights of high water and low water for fourteen of the Indian 
ports. 

To predict the tides for the India and China Seas and Australia 
we have a much more difficult thing to do than for the British 
ports. The Admiralty Tide Tables give all that is necessary for the 
British ports, practically speaking; but for other parts of the world 
generally the diurnal tide comes so much into play that we have 
exceedingly complicated action. The most complete thing would be 
a table showing the height of the water every hour of the twenty- 
four. No one has yet ventured to do that generally for all parts of 



THE TIDES 297 

the world; but for the comparatively complicated tides of the India 
Seas, the curves traced by the Tide Predictor from which is ob- 
tained the information given in these Indian tide tables, do actually 
tell the height of the water for every instant of the twenty-four 
hours. 

The mechanical method which I have utilised in this machine is 
primarily due to the Rev. F. Bashforth who, in 1845, when he was a 
Bachelor of Arts and Fellow of St. John's College, Cambridge, de- 
scribed it to Section A of the 1845 (Cambridge) meeting of the 
British Association in a communication entitled "A Description of a 
Machine for finding the Numerical Roots of Equations and tracing 
a Variety of Useful Curves," of which a short notice appears in 
the British Association Report for that year. The same subject was 
taken up by Mr. Russell in a communication to the Royal Society in 
1869, "On the Mechanical Description of Curves,"^ which contains 
a drawing showing mechanism substantially the same as that of 
the Tide Predictor. Here is the principle as embodied in No. 3 Tide 
Predictor (represented in Fig. 133, p. 296), now actually before 
you: — 

A long cord of which one end is held fixed passes over one pulley, 
under another, and so on. These eleven pulleys are all moved up and 
down by cranks, and each pulley takes in or lets out cord according 
to the direction in which it moves. These cranks are all moved by 
trains of wheels gearing into the eleven wheels fixed on this driving 
shaft. The greatest number of teeth on any wheel is 802 engaging 
with another of 423. All the other wheels have comparatively small 
numbers of teeth. The machine is finished now, except a cast-iron 
sole and cast-iron back. A flywheel of great inertia enables me to 
turn the machine fast, without jerking the pulleys, and so to run off a 
year's curve in about twenty-five minutes. This machine is arranged 
for fifteen constituents in all and besides that there is an arrange- 
ment for analysing out the long period tides. 

The following table shows how close an approximation to astro- 
nomical accuracy is given by the numbers chosen for the teeth of the 
several wheels. These numbers I have found by the ordinary arith- 
metical progress of converging fractions. 

'^Proc. Royal Society, June 17, 1869; (vol. xviii. p. 72). 



298 



KELVIN 



Tidal 


Speed in Degrees per Mean Solar Hour. 


Losses of Angle in 
Machine. 


Con- 
stitu- 
ents. 










Accurate. 


As given by Machine. 


Per Mean 
Solar Hour. 


Per 
Half 
Year. 


M, 


28°'984i042 


15X^^5 - 

2SI 


28°"984o630 


-|- 0°'00004I2 


o°-i8o 


K, 


iS°'04io686 


xsxf = 
36s 


i5°-O4i09S9 


— o°"oooo267 


o°-ii7 





i3''"94303S6 


--^ = 


i3°-943o894 


— o°-oooo538 


o°-237 


P 


I4°-95893I4 


'< - 


i4°-958904o 


-|- 0°'0000242 


o°ii9 


N 


28°-4397296 


,,802 
iSX — = 

423 


28°-4397i63 


-1- o°-ooooi33 


o°-os9 


L 


29°-S284788 


159 


29°-S283oi8 


-|- o°'oooi77 


o''■^?, 


V 


28°-si2s830 


i5X^ = 
121 


28°-si23966 


+ o''"oooi864 


o°-82 


M S 


58°'984I042 


-x^xgr- 


= S8''-98366oo 


-|- o°'ooo444 


i°-9S 


li 


27°-9682o84 


«xf = 


27°-968i27S 


-f o°"oooo8o9 


o°-36 


X 


29°-4Ss6254 


i^X*^ - 

'^^248 ~ 


29°-4S5645i 


— o°'ooooig7 


o°-o87 


Q 


i3°"39866o9 


iSX^ = 

459 


i3°-3986928 


— o°'oooo3i8 


o°-i4 



To-day (Aug. 25, 1882), a committee, consisting of only two 
members, Mr. George Darwin and Professor Adams of Cambridge, 
have been appointed, and one of their chief objects will be to exam- 
ine the long period tides. 

There is one very interesting point I said I would endeavour to 
speak of if I had time; I have not time, but still I must speak of it — 
the influence of the moon on the weather. "We almost laugh when 
we hear of the influence of the moon on the weather," Sir F. Evans 
said to me, "but there is an influence." Gales of wind are remarkably 



THE TIDES 299 

prevalent in Torres Straits and the neighbourhood about the time 
of new and full moon. This was noticed by Dr. Rattray, a surgeon 
in the navy, in connection with observations made by the surveying 
ship. Fly, during the three years 1841-44. Dr. Rattray noticed that 
at those times there was a large area of coral reef uncovered at the 
very low water of the spring tides, extending out some sixty or 
seventy miles from land. This large area becomes highly heated, 
and the great heating of that large portion of land gives rise to a 
tendency to gales at the full and change, that is at the new and full 
moon. This indirect effect of the moon upon the weather through 
the tides is exceedingly interesting; but it does not at all invalidate 
the scientific conclusion that there is no direct influence, and the 
general effect of the moon on the weather — the changes in the moon 
and the changes in the weather, and their supposed connection — 
remains a mere chimaera. 

The subject of elastic tides in which the yielding of the solid earth 
is taken into account is to be one of the primary objects of Mr. G. 
Darwin's committee. The tide-generating force which tends to pull 




Fig. 134 — Elastic Tides 

the water to and from the moon, tends to pull the earth also. 
Imagine the earth made of india-rubber and pulled out to and from 
the moon. It will be made prolate (Fig. 134) . If the earth were of 
india-rubber the tides would be nothing, the rise and fall of the 
water relatively to the solid would be practically nil. If the earth 
(as has long been a favourite hypothesis of geologists) had a thin 
shell 20 or 30 miles thick with liquid inside, there would be no such 
thing as tides of water rising and falling relatively to land, or sea- 
bottom. The earth's crust would yield to and from the moon, and the 
water would not move at all relatively to the crust. If the earth were 
even as rigid as glass all through, calculation shows that the solid 
would yield so much that the tides could only be about one third of 
what they would be if the earth were perfectly rigid. Again, if the 



300 KELVIN 

earth were two or three times as rigid as glass, about as rigid as a 
sohd globe of steel, it would still, considering its great dimensions, 
yield two or three feet to that great force, which elastic yielding 
would be enough to make the tides only two thirds of what they 
would be if the earth were perfectly rigid. Mr. G. Darwin has made 
the investigation by means of the lunar fortnightly tides, and the 
general conclusion, subject to verification, is that the earth does 
seem to yield somewhat, and may have something like the rigidity 
of a solid globe of steel. 



APPENDIX A 

[Extracts from a Lecture on "The Tides," given to the Glasgow 
Science Lectures Association, not hitherto published, and now in- 
cluded as explaining in greater detail certain paragraphs of the 
preceding Lecture. \ 

(i) Gravitation. — The great theory of gravitation put before us 
by Newton asserts that every portion of matter in the universe at- 
tracts every other portion; and that the force depends on the masses 
of the two portions considered, and on the distance between them. 
Now, the first great point of Newton's theory is, that bodies which 
have equal masses are equally attracted by any other body, a body 
of double mass experiencing double force. This may seem only what 
is to be expected. It would take more time than we have to spare 
were I to point out all that is included in this statement; but let me 
first explain to you how the motions of different kinds of matter 
depend on a property called inertia. I might show you a mass of iron 
as here. Consider that if I apply force to it, it gets into a state of 
motion; greater force applied to it, during the same time, gives it 
increased velocity, and so on. Now, instead of a mass of iron, I 
might hang up a mass of lead, or a mass of wood, to test the equality 
of the mass by the equality of the motion which is produced in the 
same time by the action of the same force, or in equal times by the 
action of equal forces. Thus, quite irrespectively of the kind of 
matter concerned, we have a test of the quantity of matter. You 
might weigh a pound of tea against a pound of brass without ever 
putting them into the balance at all. You might hang up one body by 
a proper suspension, and you might, by a spring, measure the force 
applied, first to the one body, and then to the other. If the one body 
is found to acquire equal velocity under the influence of equal force 
for equal times as compared with the other body, then the mass of 
the one is said to be equal to the mass of the other. 

301 



302 KELVIN 

I have spoken of mutual forces between any two masses. Let us 
consider the weight or heaviness of a body on the earth's surface. 
Newton explained that the attraction of the whole earth upon a body 
— ^for example, this 56 pounds mass of iron — causes its heaviness or 
weight. Well, now, take 56 pounds of iron here, and take a mass of 
lead, which, when put in the balance, is found to be of equal weight. 
You see we have quite a hew idea here. You weigh this mass of iron 
against a mass of lead, or to weigh out a commodity for sale; as, for 
instance, to weigh out pounds of tea, to weigh them with brass 
weights is to compare their gravitations towards the earth — to com- 
pare the heavinesses of the different bodies. But the first subject that 
I asked you to think of had nothing to do with heaviness. The first 
subject was the mass of the different bodies as tested by their resist- 
ance to force tending to set them in motion. I may just say that 
the property of resistance against being set into motion, and again 
against resistance to being stopped when in motion, is the property 
of matter called inertia. 

The first great point in Newton's discovery shows, then, that if 
the property of inertia is possessed to an equal degree by two dif- 
ferent substances, they have equal heaviness. One of his proofs was 
founded on the celebrated guinea and feather experiment, showing 
that the guinea and feather fall at the same rate when the resistance 
of the air is removed. Another was founded upon making pendu- 
lums of different substances — lead, iron, and wood — to vibrate, and 
observing their times of vibration. Newton thus discovered that 
bodies which have equal heaviness have equal inertia. 

The other point of the law of gravitation is, that the force between 
any two bodies diminishes as the distance increases, according to the 
law of the inverse square of the distance. That law expresses that, 
with double distance, the force is reduced one quarter, at treble 
distance the force is reduced to one-ninth part. Suppose we compare 
forces at the distance of one million miles, then again at the distance 
of two and a half million miles, we have to square the one number, 
then square the other, and find the proportion of the square of the one 
number to the square of the other. The forces are inversely as the 
squares of the distance, that is the most commonly quoted part of 
the law of gravitation; but the law is incomplete without the first 



THE TIDES 303 

part, which establishes the relation between two apparently different 
properties of matter. Newton founded this law upon a great variety 
of different natural phenomena. The motion of the planets round 
the sun, and the moon round the earth, proved that for each planet 
the force varies inversely as the square of its distance from the sun; 
and that from planet to planet the forces on equal portions of their 
masses are inversely as the squares of their distances. The last 
link in the great chain of this theory is the tides. 

(2) Tide-Generating Force. — And now we are nearly ready to 
complete the theory of tide-generating force. The first rough view of 
the case, which is not always incorrect, is that the moon attracts the 
waters of the earth towards herself and heaps them up, therefore, on 
one side of the earth. It is not so. It would be so if the earth 
and moon were at rest and prevented from falling together by a rigid 
bar or column. If the earth and moon were stuck on the two ends of 
a strong bar, and put at rest in space, then the attraction of the moon 
would draw the waters of the earth to the side of the earth next to 
the moon. But in reality things are very different from that supposi- 
tion. There is no rigid bar connecting the moon and the earth. Why 
then does not the moon fall towards the earth ? According to New- 
ton's theory, the moon is always falling towards the earth. Newton 
compared the fall of the moon, in his celebrated statement, with the 
fall of a stone at the earth's surface, as he recounted, after the fall of 
an apple from the tree, which he perceived when sitting in his gar- 
den musing on his great theory. The moon is falling towards the 
earth, and falls in an hour as far as a stone falls in a second. It 
chances that the number 60 is nearly enough, as I have said before, 
a numerical expression for the distance of the moon from the earth 
in terms of the earth's radius. It is only by that chance that the com- 
parison between the second and hour can be here introduced. Since 
there are 60 times 60 seconds in an hour, and about 60 radii of the 
earth in the distance from the moon, we are led to the comparison 
now indicated, but I am inverting the direction of Newton's com- 
parison. He found by observation that the moon falls as far in 
an hour as a stone falls in a second, and hence inferred that the force 
on the moon is a 60th of the 60th of the force per equal mass on the 
earth's surface. Then he learned from accurate observations, and 



304 KELVIN 

from the earth's dimensions, what I have mentioned as the moon's 
distance, and perceived the law of variation between the weight of a 
body at the earth's surface and the force that keeps the moon in her 
orbit. The moon in Newton's theory was always falling towards the 
earth. Why does it not come down? Can it be always falling and 
never come down ? That seems impossible. It is always falling, but 
it has also a motion perpendicular to the direction in which it is 
falling, and the result of that continual falling is simply a change 
of direction of this motion. 

It would occupy too much of our time to go into this theory. It 
is simply the dynamical theory of centrifugal force. There is a con- 
tinual falling away from the line of motion, as illustrated in a stone 
thrown from the hand describing an ordinary curve. You know that 
if a stone is thrown horizontally it describes a parabola — the stone 
falling away from the line in which it was thrown. The moon is 
continually falling away from the line in which it moves at any 
instant, falling away towards the point of the earth's centre, and 
falling away towards that point in the varying direction from itself. 
You can see it may be always falling, now from the present direc- 
tion, now from the altered direction, now from the farther altered 
direction in a further altered line; and so it may be always falling 
and never coming down. The parts of the moon nearest to the 
earth tend to fall most rapidly, the parts furthest from the earth, 
least rapidly; in its own circle, each is falling away and the result 
is as if we had the moon falling directly. 

But while the moon is always falling towards the earth, the earth 
is always falling towards the moon; and each preserves a constant 
distance, or very nearly a constant distance from the common centre 
of gravity of the two. The parts of the earth nearest to the moon are 
drawn towards the moon with more force than an equal mass at 
the average distance; the most distant parts are drawn towards the 
moon with less force than corresponds to the average distance. The 
solid mass of the earth, as a whole, experiences, according to its mass, 
a force depending on the average distance; while each portion of 
the water on the surface of the earth experiences an attractive force 
due to its own distance from the moon. The result clearly is, then, a 
tendency to protuberance towards the moon and from the moon; 



THE TIDES 305 

and thus, in a necessarily most imperfect manner, I have explained 
to you how it is that the waters are not heaped up on the side next 
the moon, but are drawn up towards the moon and left away from 
the moon so as to tend to form an oval figure. The diagram (Fig, 
125, p. 284) shows the protuberance of water towards and from the 
moon. It shows also the sun on the far side, I need scarcely say, 
with an enormous distortion of proportions, because without that it 
would be impossible in a diagram to show the three bodies. This 
illustrates the tendency of the tide-generating forces. 

(3) Elastic Tides. — But another question arises. This great force 
of gravity operating in different directions, pulling at one place, 
pressing in at another, will it not squeeze the earth out of shape? 
I perceive signs of incredulity; you think it impossible it can pro- 
duce any sensible effect. Well, I will just tell you that instead of 
being impossible, instead of it not producing any such effect, we 
have to suppose the earth to be of exceedingly rigid material, in order 
that the effect of these distorting influences on it may not mask the 
phenomenon of the tides altogether. 

There is a very favourite geological hypothesis which I have no 
doubt many here present have heard, which perhaps till this moment 
many here present have believed, but which I hope no one will go 
out of this room believing, and that is that the earth is a mere crust, 
a solid shell thirty, or forty, or fifty miles thick at the most, and that 
it is filled with molten liquid lava. This is not a supposition to be 
dismissed as absurd, as ludicrous, as absolutely unfounded and un- 
reasonable. It is a theory based on hypothesis which requires most 
careful weighing. But it has been carefully weighed and found 
wanting in conformity to the truth. On a great many different es- 
sential points it has been found at variance with the truth. One of 
these points is, that unless the material of this supposed shell were 
preternaturally rigid, were scores of times more rigid than steel, the 
shell would yield so freely to the tide-generating forces that it would 
take the figure of equilibrium, and there would be no rise and fall 
of the water, relatively to the solid land, left to show us the phe- 
nomena of the tides. 

Imagine that this (Fig. 134, p. 299) represents a solid shell with 
water outside, you can understand if the solid shell yields with suf- 



306 KELVIN 

ficiently great freedom, there will be exceedingly little tidal yielding 
left for the water to show. It may seem strange when I say that 
hard steel would yield so freely. But consider the great hardness of 
steel and the smaller hardness of india-rubber. Consider the great- 
ness of the earth, and think of a little hollow india-rubber ball, how 
freely it yields to the pressure of the hand, or even to its own weight 
when laid on a table. Now, take a great body like the earth: the 
greater the mass the more it is disposed to yield to the attraction of 
distorting forces when these forces increase with the whole mass. I 
cannot just now fully demonstrate to you this conclusion; but I say 
that a careful calculation of the forces shows that in virtue of the 
greatness of the mass it would require an enormously increased 
rigidity in order to keep in shape. So that if we take the actual di- 
mensions of the earth at forty-two million feet diameter, and the 
crust at fifty miles thick, or two hundred and fifty thousand feet, and 
with these proportions make the calculation, we find that something 
scores of times more rigid than steel would be required to keep the 
shape so well as to leave any appreciable degree of difference from 
the shape of hydrostatic equilibrium, and allow the water to indicate, 
by relative displacement, its tendency to take the figure of equilib- 
rium; that is to say, to give us the phenomena of tides. The geologi- 
cal inference from this conclusion is, that not only must we deny the 
fluidity of the earth and the assertion that it is encased by a thin 
shell, but we must say that the earth has, on the whole, a rigidity 
greater than that of a solid globe of glass of the same dimensions; 
and perhaps greater than that of a globe of steel of the same dimen- 
sions. But that it cannot be less rigid than a globe of glass, we are 
assured. It is not to be denied that there may be a very large space 
occupied by liquid. We know there are large spaces occupied by 
lava; but we do not know how large they may be, although we can 
certainly say that there are no such spaces, as can in volume be 
compared with the supposed hollow shell, occupied by liquid con- 
stituting the interior of the earth. The earth as a whole must be 
rigid, and perhaps exceedingly rigid, probably rendered more rigid 
than it is at the surface strata by the greater pressure in the greater 
depths. 



THE TIDES 307 

The phenomena of underground temperature, which led geologists 
to that supposition, are explained otherwise than by their assumption 
of a thin shell full of liquid; and further, every view we can take of 
underground temperature, in the past history of the earth, confirms 
the statement that we have no right to assume interior fluidity. 



THE EXTENT OF THE UNIVERSE 



BY 
SIMON NEWCOMB 



INTRODUCTORY NOTE 

In spite of the fertility of America in mechanical invention and applied 
science, there are few branches of pure science in which she can be 
regarded as among the leading nations. Her nearest approach to pre- 
eminence has probably been in astronomy; and in this field Simon New- 
comb was, at his death, the most distinguished figure. 

Newcomb was born in the village of Wallace, Nova Scotia, March 12, 
1835. His father, who was a teacher, gave him his elementary education; 
and at the age of eighteen we find him teaching a country school in 
Maryland. Two years later, a position as computer on the "Nautical 
Almanac" brought him to Cambridge, Mass., where he studied in Har- 
vard University till 1861, when he was appointed professor of mathe- 
matics in the United States Navy. He remained in the government 
service till he was retired as a rear admiral in 1897, having served besides 
as professor of mathematics and astronomy in Johns Hopkins University, 
Baltimore, from 1884. 

Newcomb's chief labors were in the department of mathematical 
astronomy, and were directed toward the explanation of the observed 
movements of the heavenly bodies. The difficulty and complexity of the 
calculations involved are beyond the conception of the layman; and the 
achievements which brought Newcomb honors from the learned of 
almost all civilized countries have to be taken on trust by the general. 
He had, nevertheless, an admirable power of clear exposition of those 
parts of his subject which were capable of popularization; and the accom- 
panying paper is a good example of the simple treatment of a large 
subject. 

Newcomb's interests extended beyond his special field, and he wrote 
with vigor and originality on finance and economics, and played a lead- 
ing part in the general intellectual life of his time. When he died in the 
midst of his labors on July 11, 1909, he left a place at the head of 
American science that will not easily be filled. 



THE EXTENT OF THE 
UNIVERSE 

WE cannot expect that the wisest men o£ our remotest pos- 
terity, who can base their conclusions upon thousands of 
years of accurate observation, will reach a decision on this 
subject without some measure of reserve. Such being the case, it 
might appear the dictate of wisdom to leave its consideration to some 
future age, when it may be taken up with better means of informa- 
tion than we now possess. But the question is one which will refuse 
to be postponed so long as the propensity to think of the possibilities 
of creation is characteristic of our race. The issue is not whether we 
shall ignore the question altogether, like Eve in the presence of 
Raphael; but whether in studying it we shall confine our specula- 
tions within the limits set by sound scientific reasoning. Essaying 
to do this, I invite the reader's attention to what science may suggest, 
admitting in advance that the sphere of exact knowledge is small 
compared with the possibilities of creation, and that outside this 
sphere we can state only more or less probable conclusions. 

The reader who desires to approach this subject in the most recep- 
tive spirit should begin his study by betaking himself on a clear, 
moonless evening, when he has no earthly concern to disturb the 
serenity of his thoughts, to some point where he can lie on his back 
on bench or roof, and scan the whole vault of heaven at one view. 
He can do this with the greatest pleasure and profit in late summer 
or autumn — winter would do equally well were it possible for the 
mind to rise so far above bodily conditions that the question of 
temperature should not enter. The thinking man who does this 
under circumstances most favorable for calm thought will form a new 
conception of the wonder of the universe. If summer or autumn be 
Copyright, 1906, by Harper & Brothers. 
3" 



312 NEWCOMB 

chosen, the stupendous arch of the Milky Way will pass near the 
zenith, and the constellation Lyra, led by its beautiful blue Vega of 
the first magnitude, may be not very far from that point. South of it 
will be seen the constellation Aquila, marked by the bright Altair, 
between two smaller but conspicuous stars. The bright Arcturus will 
be somewhere in the west, and, if the observation is not made too 
early in the season, Aldebaran will be seen somewhere in the east. 
When attention is concentrated on the scene the thousands of stars on 
each side of the Milky Way will fill the mind with the consciousness 
of a stupendous and all-embracing frame, beside which all human 
affairs sink into insignificance. A new idea will be formed of such 
a well-known fact of astronomy as the motion of the solar system in 
space, by reflecting that, during all human history, the sun, carrying 
the earth with it, has been flying towards a region in or just south 
of the constellation Lyra, with a speed beyond all that art can pro- 
duce on earth, without producing any change apparent to ordinary 
vision in the aspect of the constellation. Not only Lyra and Aquila, 
but every one of the thousand stars which form the framework of the 
sky, were seen by our earliest ancestors just as we see them now. 
Bodily rest may be obtained at any time by ceasing from our labors, 
and weary systems may find nerve rest at any summer resort; but I 
know of no way in which complete rest can be obtained for the 
weary soul — in which the mind can be so entirely relieved of the 
burden of all human anxiety — as by the contemplation of the spec- 
tacle presented by the starry heavens under the conditions just 
described. As we make a feeble attempt to learn what science can 
tell us about the structure of this starry frame, I hope the reader will 
allow me to at least fancy him contemplating it in this way. 

The first question which may suggest itself to the inquiring reader 
is: How is it possible by any methods of observation yet known to 
the astronomer to learn anything about the universe as a whole? 
We may commence by answering this question in a somewhat com- 
prehensive way. It is possible only because the universe, vast though 
it is, shows certain characteristics of a unified and bounded whole. 
It is not a chaos, it is not even a collection of things, each of which 
came into existence in its own separate way. If it were, there would 
be nothing in common between two widely separate regions of the 



THE EXTENT OF THE UNIVERSE 313 

universe. But, as a matter of fact, science shows unity in the whole 
structure, and diversity only in details. The Milky Way itself will 
be seen by the most ordinary observer to form a single structure. 
This structure is, in some sort, the foundation on which the universe 
is built. It is a girdle which seems to span the whole of creation, so 
far as our telescopes have yet enabled us to determine what creation 
is; and yet it has elements of similarity in all its parts. What has 
yet more significance, it is in some respects unlike those parts of the 
universe which lie without it, and even unlike those which lie in that 
central region within it where our system is now situated. The 
minute stars, individually far beyond the limits of visibility to the 
naked eye, which form its cloudlike agglomerations, are found to be 
mostly bluer in color, from one extreme to the other, than the gen- 
eral average of the stars which make up the rest of the universe. 

In the preceding essay on the structure of the universe, we have 
pointed out several features of the universe showing the unity of the 
whole. We shall now bring together these and other features with 
a view of showing their relation to the question of the extent of the 
universe. 

The Milky Way being in a certain sense the foundation on which 
the whole system is constructed, we have first to notice the symmetry 
of the whole. This is seen in the fact that a certain resemblance is 
found in any two opposite regions of the sky, no matter where we 
choose them. If we take them in the Milky Way, the stars are more 
numerous than elsewhere; if we take opposite regions in or near the 
Milky Way, we shall find more stars in both of them than elsewhere; 
if we take them in the region anywhere around the poles of the 
Milky Way, we shall find fewer stars, but they will be equally nu- 
merous in each of the two regions. We infer from this that whatever 
cause determined the number of the stars in space was of the same 
nature in every two antipodal regions of the heavens. 

Another unity marked with yet more precision is seen in the 
chemical elements of which stars are composed. We know that the 
sun is composed of the same elements which we find on the earth 
and into which we resolve compounds in our laboratories. These 
same elements are found in the most distant stars. It is true that 
some of these bodies seem to contain elements which we do not find 



314 NEWCOMB 

on earth. But as these unknown elements are scattered from one 
extreme of the universe to the other, they only serve still further to 
enforce the unity which runs through the whole. The nebulae are 
composed, in part at least, of forms of matter dissimilar to any with 
which we are acquainted. But, different though they may be, they 
are alike in their general character throughout the whole field we 
are considering. Even in such a feature as the proper motions of the 
stars, the same unity is seen. The reader doubtless knows that each 
of these objects is flying through space on its own course with a speed 
comparable with that of the earth around the sun. These speeds 
range from the smallest limit up to more than one hundred miles 
a second. Such diversity might seem to detract from the unity of 
the whole; but when we seek to learn something definite by taking 
their average, we find this average to be, so far as can yet be deter- 
mined, much the same in opposite regions of the universe. Quite 
recently it has become probable that a certain class of very bright 
stars known as Orion stars — because there are many of them in the 
most brilliant of our constellations — which are scattered along the 
whole course of the Milky Way, have one and all, in the general 
average, slower motions than other stars. Here again we have a 
definable characteristic extending through the universe. In drawing 
attention to these points of similarity throughout the whole universe, 
it must not be supposed that we base our conclusions directly upon 
them. The point they bring out is that the universe is in the nature 
of an organized system; and it is upon the fact of its being such a 
system that we are able, by other facts, to reach conclusions as to its 
structure, extent, and other characteristics. 

One of the great problems connected with the universe is that of 
its possible extent. How far away are the stars? One of the unities 
which we have described leads at once to the conclusion that the 
stars must be at very different distances from us; probably the more 
distant ones are a thousand times as far as the nearest; possibly even 
farther than this. This conclusion may, in the first place, be based 
on the fact that the stars seem to be scattered equally throughout 
those regions of the universe which are not connected with the 
Milky Way. To illustrate the principle, suppose a farmer to sow a 
wheat field of entirely unknown extent with ten bushels of wheat. 



THE EXTENT OF THE UNIVERSE 315 

We visit the field and wish to have some idea of its acreage. We 
may do this if we know how many grains of wheat there are in the 
ten bushels. Then we examine a space two or three feet square in 
any part of the field and count the number of grains in that space. 
If the wheat is equally scattered over the whole field, we find its 
extent by the simple rule that the size of the field bears the same 
proportion to the size of the space in which the count was made 
that the whole number of grains in the ten bushels sown bears to 
the number of grains counted. If we find ten grains in a square 
foot, we know that the number of square feet in the whole field is 
one-tenth that of the number of grains sown. So it is with the uni- 
verse of stars. If the latter are sown equally through space, the 
extent of the space occupied must be proportional to the number of 
stars which it contains. 

But this consideration does not tell us anything about the actual 
distance of the stars or how thickly they may be scattered. To do 
this we must be able to determine the distance of a certain number of 
stars, just as we suppose the farmer to count the grains in a certain 
small extent of his wheat field. There is only one way in which we 
can make a definite measure of the distance of any one star. As the 
earth swings through its vast annual circuit round the sun, the direc- 
tion of the stars must appear to be a little different when seen from 
one extremity of the circuit than when seen from the other. This 
difference is called the parallax of the stars; and the problem of 
measuring it is one of the most delicate and difficult in the whole 
field of practical astronomy. 

The nineteenth century was well on its way before the instru- 
ments of the astronomer were brought to such perfection as to admit 
of the measurement. From the time of Copernicus to that of Bessel 
many attempts had been made to measure the parallax of the stars, 
and more than once had some eager astronomer thought himself 
successful. But subsequent investigation always showed that he 
had been mistaken, and that what he thought was the effect of 
parallax was due to some other cause, perhaps the imperfections of 
his instrument, perhaps the effect of heat and cold upon it or upon 
the atmosphere through which he was obliged to observe the star, 
or upon the going of his clock. Thus things went on until 1837, 



3l6 NEWCOMB 

when Bessel announced that measures with a heliometer — the most 
refined instrument that has ever been used in measurement — showed 
that a certain star in the constellation Cygnus had a parallax of one- 
third of a second. It may be interesting to give an idea of this 
quantity. Suppose one's self in a house on top of a mountain looking 
out of a window one foot square, at a house on another mountain 
one hundred miles away. One is allowed to look at that distant house 
through one edge of the pane of glass and then through the opposite 
edge; and he has to determine the change in the direction of the dis- 
tant house produced by this change of one foot in his own position. 
From this he is to estimate how far off the other mountain is. To do 
this, one would have to measure just about the amount of parallax 
that Bessel found in his star. And yet this star is among the few 
nearest to our system. The nearest star of all, Alpha Centauri, visible 
only in latitudes south of our middle ones, is perhaps half as far as 
Bessel's star, while Sirius and one or two others are nearly at the 
same distance. About loo stars, all told, have had their parallax 
measured with a greater or less degree of probability. The work is 
going on from year to year, each successive astronomer who takes 
it up being able, as a general rule, to avail himself of better instru- 
ments or to use a better inethod. But, after all, the distances of even 
some of the loo stars carefully measured must still remain quite 
doubtful. 

Let us now return to the idea of dividing the space in which the 
universe is situated into concentric spheres drawn at various dis- 
tances around our system as a centre. Here we shall take as our 
standard a distance 400,000 times that of the sun from the earth. 
Regarding this as a unit, we imagine ourselves to measure out in any 
direction a distance twice as great as this — then another equal dis- 
tance, making one three times as great, and so indefinitely. We then 
have successive spheres of which we take the nearer one as the unit. 
The total space filled by the second sphere will be 8 times the unit; 
that of the third space 27 times, and so on, as the cube of each 
distance. Since each sphere includes all those within it, the volume 
of space between each two spheres will be proportional to the differ- 
ence of these numbers — that is, to i, 7, 19, etc. Comparing these 
volumes with the number of stars probably within them, the general 



THE EXTENT OF THE UNIVERSE $IJ 

result up to the present time is that the number of stars in any o£ 
these spheres will be about equal to the units o£ volume which they 
comprise, when we take for this unit the smallest and innermost of 
the spheres, having a radius 400,000 times the sun's distance. We are 
thus enabled to form some general idea of how thickly the stars are 
sown through space. We cannot claim any numerical exactness for 
this idea, but in the absence of better methods it does afford us some 
basis for reasoning. 

Now we can carry on our computation as we supposed the farmer 
to measure the extent of his wheat field. Let us suppose that there 
are 125,000,000 stars in the heavens. This is an exceedingly rough 
estimate, but let us make the supposition for the time being. Accept- 
ing the view that they are nearly equally scattered throughout space, 
it will follow that they must be contained within a volume equal 
to 125,000,000 times the sphere we have taken as our unit. We find 
the distance of the surface of this sphere by extracting the cube root 
of this number, which gives us 500. We may, therefore, say, as the 
result of a very rough estimate, that the number of stars we have 
supposed would be contained within a distance found by multiply- 
ing 400,000 times the distance of the sun by 500; that is, that they are 
contained within a region whose boundary is 200,000,000 times the 
distance of the sun. This is a distance through which light would 
travel in about 3,300 years. 

It is not impossible that the number of stars is much greater than 
that we have supposed. Let us grant that there are eight times as 
many, or 1,000,000,000. Then we should have to extend the boundary 
of our universe twice as far, carrying it to a distance which light 
would require 6,600 years to travel. 

There is another method of estimating the thickness with which 
stars are sown through space, and hence the extent of the universe, 
the result of which will be of interest. It is based on the proper 
motion of the stars. One of the greatest triumphs of astronomy of 
our time has been the measurement of the actual speed at which 
many of the stars are moving to or from us in space. These measures 
are made with the spectroscope. Unfortunately, they can be best 
made only on the brighter stars — ^becoming very difficult in the case 
of stars not plainly visible to the naked eye. Still the motions of 



3l8 NEWCOMB 

several hundreds have been measured and the number is constantly 
increasing. 

A general result of all these measures and of other estimates may 
be summed up by saying that there is a certain average speed with 
which the individual stars move in space; and that this average is 
about twenty miles per second. We are also able to form an estimate 
as to what proportion of the stars move with each rate of speed from 
the lowest up to a limit which is probably as high as 150 miles per 
second. Knowing these proportions we have, by observation of the 
proper motions of the stars, another method of estimating how 
thickly they are scattered in space; in other words, what is the volume 
of space which, on the average, contains a single star. This method 
gives a thickness of the stars greater by about twenty-five per cent, 
than that derived from the measures of parallax. That is to say, a 
sphere like the second we have proposed, having a radius 800,000 
times the distance of the sun, and therefore a diameter 1,600,000 
times this distance, would, judging by the proper motions, have ten 
or twelve stars contained within it, while the measures of parallax 
only show eight stars within the sphere of this diameter having the 
sun as its centre. The probabilities are in favor of the result giving 
the greater thickness of the stars. But, after all, the discrepancy does 
not change the general conclusion as to the limits of the visible 
universe. If we cannot estimate its extent with the same certainty 
that we can determine the size of the earth, we can still form a 
general idea of it. 

The estimates we have made are based on the supposition that the 
stars are equally scattered in space. We have good reason to believe 
that this is true of all the stars except those of the Milky Way. But, 
after all, the latter probably includes half the whole number of 
stars visible with a telescope, and the question may arise whether our 
results are seriously wrong from this cause. This question can best 
be solved by yet another method of estimating the average distance 
of certain classes of stars. 

The parallaxes of which we have heretofore spoken consist in the 
change in the direction of a star produced by the swing of the earth 
from one side of its orbit to the other. But we have already remarked 
that our solar system, with the earth as one of its bodies, has been 



THE EXTENT OF THE UNIVERSE 319 

journeying straightforward through space during all historic times. 
It follows, therefore, that we are continually changing the position 
from which we view the stars, and that, if the latter were at rest, 
we could, by measuring the apparent speed with which they are 
moving in the opposite direction from that of the earth, determine 
their distance. But since every star has its own motion, it is impos- 
sible, in any one case, to determine how much of the apparent motion 
is due to the star itself, and how much to the motion of the solar 
system through space. Yet, by taking general averages among groups 
of stars, most of which are probably near each other, it is possible 
to estimate the average distance by this method. When an attempt 
is made to apply it, so as to obtain a definite result, the astronomer 
finds that the data now available for the purpose are very deficient. 
The proper motion of a star can be determined only by comparing 
its observed position in the heavens at two widely separate epochs. 
Observations of sufficient precision for this purpose were commenced 
about 1750 at the Greenwich Observatory, by Bradley, then Astron- 
omer Royal of England. But out of 3,000 stars which he determined, 
only a few are available for the purpose.- Even since his time, the 
determinations made by each generation of astronomers have not 
been sufficiently complete and systematic to furnish the material for 
anything like a precise determination of the proper motions of stars. 
To determine a single position of any one star involves a good deal 
of computation, and if we reflect that, in order to attack the problem 
in question in a satisfactory way, we should have observations of 
1,000,000 of these bodies made at intervals of at least a considerable 
fraction of a century, we see what an enormous task the astronomers 
dealing with this problem have before them, and how imperfect must 
be any determination of the distance of the stars based on our motion 
through space. So far as an estimate can be made, it seems to agree 
fairly well with the results obtained by the other methods. Speaking 
roughly, we have reason, from the data so far available, to believe 
that the stars of the Milky Way are situated at a distance between 
100,000,000 and 200,000,000 times the distance of the sun. At dis- 
tances less than this it seems likely that the stars are distributed 
through space with some approach to uniformity. We may state as 
a general conclusion, indicated by several methods of making the 



320 NEWCOMB 

estimate, that nearly all the stars which we can see with our tele- 
scopes are contained within a sphere not likely to be much more than 
200,000,000 times the distance of the sun. 

The inquiring reader may here ask another question. Granting 
that all the stars we can see are contained within this limit, may 
there not be any number of stars outside the limit which are invisible 
only because they are too far away to be seen ? 

This question may be answered quite definitely if we grant that 
light from the most distant stars meets with no obstruction in reach- 
ing us. The most conclusive answer is afforded by the measure of 
starlight. If the stars extended out indefinitely, then the number of 
those of each order of magnitude would be nearly four times that 
of the magnitude next brighter. For example, we should have nearly 
four times as many stars of the sixth magnitude as of the fifth; nearly 
four times as many of the seventh as of the sixth, and so on indefi- 
nitely. Now, it is actually found that while this ratio of increase is 
true for the brighter stars, it is not so for the fainter ones, and that 
the increase in the number of the latter rapidly falls off when we 
make counts of the fainter telescopic stars. In fact, it has long been 
known that, were the universe infinite in extent, and the stars equally 
scattered through all space, the whole heavens would blaze with 
the light of countless millions of distant stars separately invisible 
even with the telescope. 

The only way in which this conclusion can be invalidated is by 
the possibility that the light of the stars is in some way extinguished 
or obstructed in its passage through space. A theory to this effect 
was propounded by Struve nearly a century ago, but it has since 
been found that the facts as he set them forth do not justify the con- 
clusion, which was, in fact, rather hypothetical. The theories of 
modern science converge towards the view that, in the pure ether of 
space, no single ray of light can ever be lost, no matter how far it 
may travel. But there is another possible cause for the extinction 
of light. During the last few years discoveries of dark and there- 
fore invisible stars have been made by means of the spectroscope 
with a success which would have been quite incredible a very few 
years ago, and which, even to-day, must excite wonder and admira- 
tion. The general conclusion is that, besides the shining stars which 



THE EXTENT OF THE UNIVERSE 321 

exist in space, there may be any number of dark ones, forever invis- 
ible in our telescopes. May it not be that these bodies are so numer- 
ous as to cut off the light which we would otherwise receive from 
the more distant bodies of the universe ? It is, of course, impossible 
to answer this question in a positive way, but the probable conclusion 
is a negative one. We may say with certainty that dark stars are not 
so numerous as to cut off any important part of the light from the 
stars of the Milky Way, because, if they did, the latter would not be 
so clearly seen as it is. Since we have reason to believe that the 
Milky Way comprises the more distant stars of our system, we may 
feel fairly confident that not much light can be cut off by dark 
bodies from the most distant region to which our telescopes can 
penetrate. Up to this distance we see the stars just as they are. Even 
within the limit of the universe as we understand it, it is Ukely that 
more than one-half the stars which actually exist are too faint to be 
seen by human vision, even when armed with the most powerful 
telescopes. But their invisibility is due only to their distance and the 
faintness of their intrinsic light, and not to any obstructing agency. 
The possibility of dark stars, therefore, does not invalidate the 
general conclusions at which our survey of the subject points. The 
universe, so far as we can see it, is a bounded whole. It is sur- 
rounded by an immense girdle of stars, which, to our vision, appears 
as the Milky Way. While we cannot set exact limits to its distance, 
we may yet confidently say that it is bounded. It has uniformities 
running through its vast extent. Could we fly out to distances equal 
to that of the Milky Way, we should find comparatively few stars 
beyond the limits of that girdle. It is true that we cannot set any 
definite limit and say that beyond this nothing exists. What we can 
say is that the region containing the visible stars has some approxi- 
mation to a boundary. We may fairly anticipate that each successive 
generation of astronomers, through coming centuries, will obtain a 
little more light on the subject — will be enabled to make more defi- 
nite the boundaries of our system of stars, and to draw more and 
more probable conclusions as to the existence or non-existence of any 
object outside of it. The wise investigator of to-day will leave to 
them the task of putting the problem into a more positive shape. 



GEOGRAPHICAL EVOLUTION 



BY 

SIR ARCHIBALD GEIKIE 



INTRODUCTORY NOTE 

Sir Archibald Geikie was born in Edinburgh, Scotland, in 1835, and 
educated at the university in that city. He became a member of the 
Geological Survey of Scotland, of which, in 1867, he was made director; 
and from 1871 to 1882 was Murchison professor of geology and min- 
eralogy in his own university. At the latter date he resigned to take up 
the general directorship of the Geological Survey of the United Kingdom, 
which he held till 1901. He has been an extremely productive investi- 
gator, and his achievements have brought him many distinctions from 
learned societies at home and abroad, including the honor of knighthood, 
the presidency of the British Association, and the secretaryship of the 
Royal Society. 

Among his more important writings may be mentioned "The Scenery 
of Scotland, Viewed in Connection With its Physical Geography" (1865), 
"Text-book of Geology" (1882), "Ancient Volcanoes of Britain" (1897), 
"Types of Scenery and Their Influence on Literature" (1898), and 
"Landscape in History" (1905). 

The following paper on "Geographical Evolution," published among 
his "Geological Sketches at Home and Abroad" (1882), might be entitled 
with less ambiguity "Geological Evolution," since it is in this sense 
rather than in the broader modern signification that the word "geographi- 
cal" is employed throughout the essay. 

The interest in literature indicated by the names of several of Geikie's 
books has not been without influence on his writings, for there are few 
scientific authors to-day who can command so fascinating a style. His 
power of exposition is shown by the skill with which, in the present 
paper, he maps out in comparatively few pages, the views of his school 
as to the geological history of the earth. 



GEOGRAPHICAL EVOLUTION^ 

IN the quaint preface to his Navigations and Voyages of the 
English Nation, Hakluyt calls geography and chronology "the 
sunne and moone, the right eye and the left of all history." 
The position thus claimed for geography three hundred years ago by 
the great English chronicler was not accorded by his successors, and 
has hardly been admitted even now. The functions of the geog- 
rapher and the traveller, popularly assumed to be identical, have been 
supposed to consist in descriptions of foreign countries, their climate, 
productions, and inhabitants, bristling on the one hand with dry 
statistics, and relieved on the other by as copious an introduction as 
may be of stirring adventure and personal anecdote. There has 
indeed been much to justify this popular assumption. It was not 
until the key-note of its future progress vvas struck by Karl Ritter, 
within the present century, that geography advanced beyond the 
domain of travellers' tales and desultory observation into that o£ 
orderly, methodical, scientific progress. This branch of inquiry, 
however, is now no longer the pursuit of mere numerical statistics, 
nor the chronicle of marvellous and often questionable adventures 
by flood and fell. It seeks to present a luminous picture of the 
earth's surface, its various forms of configuration, its continents, 
islands, and oceans, its mountains, valleys, and plains, its rivers and 
lakes, its climates, plants, and animals. It thus endeavours to pro- 
duce a picture which shall not be one of mere topographical detail. 
It ever looks for a connection between scattered facts, tries to ascer- 
tain the relations which subsist between the different parts of the 
globe, their reactions on each other and the function of each in the 
general economy of the whole. Modern geography studies the dis- 
tribution of vegetable and animal life over the earth's surface, with 
the action and reaction between it and the surrounding inorganic 

' A Lecture delivered at the Evening Meeting of the Royal Geographical Society, 
24th March, 1879. 

325 



326 GEIKIE 

world. It traces how man, alike unconsciously and knowingly, has 
changed the face of nature, and how, on the other hand, the con- 
ditions of his geographical environment have moulded his own 
progress. 

With these broad aims geography comes frankly for assistance to 
many different branches of science. It does not, however, claim in 
any measure to occupy their domain. It brings to the consideration 
of their problems a central human interest in which these sciences 
are sometimes apt to be deficient; for it demands first of all to know 
how the problems to be solved bear upon the position and history 
of man and of this marvellously-ordered world wherein he finds 
himself undisputed lord. Geography freely borrows from meteor- 
ology, physics, chemistry, geology, zoology, and botany; but the debt 
is not all on one side. Save for the impetus derived from geograph- 
ical research, many of these sciences would not be in their present 
advanced condition. They gain in vast augmentation of facts, and 
may cheerfully lend their aid in correlating these for geographical 
requirements. 

In no respect does modern geography stand out more prominently 
than in the increased precision and fulness of its work. It has fitted 
out exploratory expeditions, and in so doing has been careful to see 
them provided with the instruments and apparatus necessary to 
enable them to contribute accurate and definite results. It has guided 
and fostered research, and has been eager to show a generous appre- 
ciation of the labours of those by whom our knowledge of the earth 
has been extended. Human courage and endurance are not less 
enthusiastically applauded than they once were; but they must be 
united to no common powers of observation before they will now 
raise a traveller to the highest rank. When we read a volume of 
recent travel, while warmly appreciating the spirit of adventure, 
fertility of resource, presence of mind, and other moral qualities of 
its author, we instinctively ask ourselves, as we close its pages, what 
is the sum of its additions to our knowledge of the earth ? From the 
geographical point of view — and it is to this point alone that these 
remarks apply — we must rank an explorer according to his success 
in widening our knowledge and enlarging our views regarding the 
aspects of nature. 



GEOGRAPHICAL EVOLUTION 327 

The demands of modern geography are thus becoming every year 
more exacting. It requires more training in its explorers abroad, 
more knowledge on the part of its readers at home. The days are 
drawing to a close when one can gain undying geographical renown 
by struggling against man and beast, fever and hunger and drought, 
across some savage and previously unknown region, even though 
little can be shown as the outcome of the journey. All honour to the 
pioneers by whom this first exploratory work has been so nobly 
done! They will be succeeded by a race that will find its laurels 
more difficult to win — a race from which more will be expected, and 
which will need to make up in the variety, amount, and value of its 
detail, what it lacks in the freshness of first glimpses into new lands. 

With no other science has geography become more intimately 
connected than with geology, and the connection is assuredly des- 
tined to become yet deeper and closer. These two branches of 
human knowledge are, to use Hakluyt's phrase, "the sunne and 
moone, the right eye and the left," of all fruitful inquiry into the 
character and history of the earth's surface. As it is impossible to 
understand the genius and temperament of a people, its laws and 
institutions, its manners and customs, its buildings and its industries, 
unless we trace back the history of that people, and mark the rise 
and effect of each varied influence by which its progress has been 
moulded in past generations; so it is clear that our knowledge of 
the aspect of a continent, its mountains and valleys, rivers and 
plains, and all its surface-features, cannot be other than singularly 
feeble and imperfect, unless we realise what has been the origin of 
these features. The land has had a history, not less than the human 
races that inhabit it. 

One can hardly consider attentively the future progress of geog- 
raphy without being convinced that in the wide development yet in 
store for this branch of human inquiry, one of its main lines of 
advance must be in the direction of what may be termed geographi- 
cal evolution. The geographer will no longer be content to take 
continents and islands, mountain chains and river valleys, table-lands 
and plains, as initial or aboriginal outlines of the earth's surface. He 
will insist on knowing what the geologist can tell him regarding the 
growth of these outlines. He will try to trace out the gradual evolu- 



328 GEIKIE 

tion of a continent, and may even construct maps to show its succes- 
sive stages of development. At the same time, he will seek for infor- 
mation regarding the history of the plants and animals of the region, 
and may find much to reward his inquiry as to the early migrations 
of the fauna and flora, including those even of man himself. Thus 
his pictures of the living world of to-day, as they become more 
detailed and accurate, will include more and more distinctly a back- 
ground of bygone geographical conditions, out of which, by con- 
tinuous sequence, the present conditions will be shown to have arisen. 

I propose this evening to sketch in mere outline the aspects of one 
side of this evolutional geography. I wish to examine, in the first 
place, the evidence whereby we establish the fundamental fact that 
the present surface of any country or continent is not that which it 
has always borne, and the data by which we may trace backward the 
origin of the land; and, in the second place, to consider, by way of 
illustration, some of the more salient features in the gradual growth 
of the framework of Europe. 

The first of these two divisions of the subject deals with general 
principles, and may be conveniently grouped into two parts: 1st, The 
Materials of the Land. 2d, The Building of the Land. 

L General Principles of Continental Evolution, 
i. — The Materials of the Land. 

Without attempting to enter into detailed treatment of this branch 
of the subject, we may, for the immediate purpose in view, content 
ourselves with the broad, useful classification of the materials of the 
land into two great series — Fragmental and Crystalline. 

§ I. Fragmental. — A very cursory examination of rocks in almost 
any part of the world suffices to show that by far the larger portion 
of them consists of compacted fragmentary materials. Shales, sand- 
stones, and conglomerates, in infinite variety of texture and colour, 
are piled above each other to form the foundation of plains and the 
structure of mountains. Each of these rocks is composed of distinct 
particles, worn by air, rain, frost, springs, rivers, glaciers, or the sea, 
from previously existing rocks. They are thus derivative formations, 
and their source, as well as their mode of origin, can be determined. 



GEOGRAPHICAL EVOLUTION 329 

Their component grains are for the most part rounded, and bear 
evidence of having been rolled about in water. Thus we easily and 
rapidly reach a first and fundamental conclusion — that the substance 
of the main part of the solid land has been originally laid down and 
assorted under water. 

The mere extent of the area covered by these water-formed rocks 
would of itself suggest that they must have been deposited in the 
sea. We cannot imagine rivers or lakes of magnitude sufficient to 
have spread over the sites of the present continents. The waters of 
the ocean, however, may easily be conceived to have rolled at differ- 
ent times over all that is now dry land. The fragmental rocks con- 
tain, indeed, within themselves proof that they were mainly of 
marine, and not of lacustrine or fluviatile origin. They have pre- 
served in abundance the remains of foraminifera, corals, crinoids, 
molluscs, annelides, crustaceans, fishes, and other organisms of 
undoubtedly marine habitat, which must have lived and died in the 
places where their traces remain still visible. 

But not only do these organisms occur scattered through sedi- 
mentary rocks; they actually themselves form thick masses of min- 
eral matter. The Carboniferous or Mountain Limestone of Central 
England and Ireland, for example, reaches a thickness of from 2,000 
to 3,000 feet, and covers thousands of square miles of surface. Yet it 
is almost entirely composed of congregated stems and joints and 
plates of crinoids, with foraminifera, corals, bryozoans, brachiopods, 
lamellibranchs, gasteropods, fish-teeth, and other unequivocally 
marine organisms. It must have been for ages the bottom of a clear 
sea, over which generation after generation lived and died, until 
their accumulated remains had gathered into a deep and compact 
sheet of rock. From the internal evidence of the stratified formations 
we thus confidently announce a second conclusion — that a great 
portion of the solid land consists of materials which have been laid 
down on the floor of the sea. 

From these familiar and obvious deductions we may proceed fur- 
ther to inquire under what conditions these marine formations, 
spreading so widely over the land, were formed. According to a 
popular belief, shared in perhaps by not a few geologists, land and 
sea have been continually changing places. It is supposed that while, 



330 GEIKIE 

on the one hand, there is no part of a continent over which sea 
waves may not have rolled, so, on the other hand, there is no lonely 
abyss of the ocean where a wide continent may not have bloomed. 
That this notion rests upon a mistaken interpretation of the facts 
may be shown from an examination — (i) of the rocks of the land, 
and (2) of the bottom of the present ocean. 

(i) Among the thickest masses of sedimentary rock — those of the 
ancient palaeozoic systems — no features recur more continually than 
alterations of different sediments, and surfaces of rock covered with 
well-preserved ripple-marks, trails and burrows of annelides, poly- 
gonal and irregular desiccation marks, like the cracks at the bottom 
of a sun-dried muddy pool. These phenomena unequivocally point 
to shallow and even littoral waters. They occur from bottom to top 
of deposits which reach a thickness of several thousand feet. They 
can be interpreted only in one way, viz., that their deposition began 
in shallow water; that during their formation the area of deposit 
gradually subsided for thousands of feet; yet that the rate of accumu- 
lation of sediment kept pace on the whole with this depression; and 
hence, that the original shallow water character of the deposits 
remained, even after the original sea bottom had been buried under 
a vast mass of sedimentary matter. Now, if this explanation be true, 
even for the enormously thick and comparatively uniform systems 
of older geological periods, the relatively thin and much more varied 
stratified groups of later date can offer no difficulty. In short, the 
more attentively the stratified rocks of the crust of the earth are 
studied, the more striking becomes the absence of any deposits among 
them which can legitimately be considered those of a deep sea. They 
have all been deposited in comparatively shallow water. 

The same conclusion may be arrived at from a consideration of 
the circumstances under which the deposition must have taken place. 
It is evident that the sedimentary rocks of all ages have been derived 
from degradation of land. The gravel, sand, and mud, of which they 
consist, existed previously as part of mountains, hills, or plains. 
These materials carried down to the sea would arrange themselves 
there as they do still, the coarser portions nearest the shore, the finer 
silt and mud farthest from it. From the earliest geological times the 
great area of deposit has been, as it still is, the marginal belt of sea 



GEOGRAPHICAL EVOLUTION 33 1 

floor skirting the land. It is there that nature has always strewn "the 
dust of continents to be." The decay of old rocks has been unceas- 
ingly in progress on the land, and the building up of new rocks has 
been unintermittently going on underneath the adjoining sea. The 
two phenomena are the complementary sides of one process, which 
belongs to the terrestrial and shallow oceanic parts of the earth's sur- 
face and not to the wide and deep ocean basins. 

(2) Recent explorations of the bottom of the deep sea all over the 
world have brought additional light to this question. No part of the 
results obtained by the Challenger Expedition has a profounder 
interest for geologists and geographers than the proof which they 
furnish that the floor of the ocean basins has no real analogy among 
the sedimentary formations that form most of the framework of the 
land. We now know by actual dredging and inspection that the 
ordinary sediment washed off the land sinks to the sea bottom before 
it reaches the deeper abysses, and that, as a rule, only the finer 
particles are carried more than a few score of miles from the shore. 
Instead of such sandy and pebbly material as we find so largely 
among the sedimentary rocks of the land, wide tracts of the sea 
bottom at great depths are covered with various kinds of organic 
ooze, composed sometimes of minute calcareous foraminifera, some- 
times of siliceous radiolaria or diatoms. Over other areas vast sheets 
of clay extend, derived apparently from the decomposition of vol- 
canic detritus, of which large quantities are floated away from vol- 
canic islands, and much of which may be produced by submarine 
volcanoes. On the tracts farthest removed from any land the sedi- 
ment seems to settle scarcely so rapidly as the dust that gathers over 
the floor of a deserted hall. Mr. Murray of the Challenger staff, has 
described how from these remote depths large numbers of shark's 
teeth and ear-bones of whales were dredged up. We cannot suppose 
the number of sharks and whales to be much greater in these regions 
than in others where their relics were found much less plentifully. 
The explanation of the abundance of their remains was supplied by 
their varied condition of decay and preservation. Some were com- 
paratively fresh, others had greatly decayed, and were incrusted with 
or even completely buried in a deposit of earthy manganese. Yet the 
same cast of the dredge brought up these different stages of decay 



332 GEIKIE 

from the same surface of the sea floor. While generation after gener- 
ation of sea creatures drops its bones to the bottom, now here, now 
there, so exceedingly feeble is the rate of deposit of sediment that they 
lie uncovered, mayhap for centuries, so that the remains which sink 
to-day may lie side by side with the mouldered and incrusted bones 
that found their way to the bottom hundreds of years ago. 

Another striking indication of the very slow rate at which sedi- 
mentation takes place in these abysses has also been brought to notice 
by Mr. Murray. In the clay from the bottom he found numerous 
minute spherical granules of native iron, which, as he suggests, are 
almost certainly of meteoric origin — fragments of those falling stars 
which, coming to us from planetary space, burst into fragments when 
they rush into the denser layers of our atmosphere. In tracts where 
the growth of silt upon the sea floor is excessively tardy, the fine 
particles, scattered by the dissipation of these meteorites, may remain 
in appreciable quantity. In this case, again, it is not needful to sup- 
pose that meteorites have disappeared over these ocean depths more 
numerously than over other parts of the earth's surface. The iron 
granules have no doubt been as plentifully showered down else- 
where, though they cannot be so readily detected in accumulating 
sediment. I know no recent observation in physical geography more 
calculated to impress deeply the imagination than the testimony of 
this presumably meteoric iron from the most distant abysses of the 
ocean. To be told that mud gathers on the floor of these abysses at 
an extremely slow rate conveys but a vague notion of the tardiness 
of the process. But to learn that it gathers so slowly, that the very 
star-dust which falls from outer space forms an appreciable part 
of it, brings home to us, as hardly anything else could do, the idea 
of undisturbed and excessively slow accumulation. 

From all this evidence we may legitimately conclude that the 
present land of the globe, though formed in great measure of marine 
formations, has never lain under the deep sea; but that its site must 
always have been near land. Even its thick marine limestones are 
the deposits of comparatively shallow water. Whether or not any 
trace of aboriginal land may now be discoverable, the characters of 
the most unequivocally marine formations bear emphatic testimony 
to this proximity of a terrestrial surface. The present continental 



GEOGRAPHICAL EVOLUTION 333 

ridges have probably always existed in some form, and as a corollary 
we may infer that the present deep ocean basins likewise date from 
the remotest geological antiquity. 

§ 2. Crystalline. — While the greater part of the framework of the 
land has been slowly built up of sedimentary materials, it is abun- 
dantly varied by the occurrence of crystalline masses, many of which 
have been injected in a molten condition into rents underground, or 
have been poured out in lava streams at the surface. 

Without entering at all into geological detail, it will be enough for 
the present purpose to recognise the characters and origin of two 
great types of crystalline material which have been called respectively 
the Eruptive and Metamorphic. 

(a) Eruptive. — As the name denotes, eruptive or igneous rocks 
have been ejected from the heated interior of the earth. In a modern 
volcano lava ascends the central funnel, and issuing from the lip of 
the crater or from lateral fissures pours down the slopes of the cone 
in sheets of melted rock. The upper surface of the lava column 
within the volcano is kept in constant ebullition by the rise of steam 
through its mass. Every now and then a vast body of steam rushes 
out with a terrific explosion, scattering the melted lava into impal- 
pable dust, and filling the air with ashes and stones, which descend 
in showers upon the surrounding country. At the surface, therefore, 
igneous rocks appear, partly as masses of congealed lava, and partly 
as more or less consolidated sheets of dust and stones. But beneath 
the surface there must be a downward prolongation of the lava 
column, which no doubt sends out veins into rents of the subter- 
ranean rocks. We can suppose that the general aspect of the lava 
which consolidates at some depth will differ from that which solidi- 
fies above ground. 

As a result of the revolutions which the crust of the earth has 
undergone, the roots of many ancient volcanoes have been laid bare. 
We have been, as it were, admitted into the secrets of these sub- 
terranean laboratories of nature, and have learned much regarding 
the mechanism of volcanic action which we could never have dis- 
covered from any modern volcano. Thus, while on the one hand we 
meet with beds of lava and consolidated volcanic ashes which were 
undoubtedly erupted at the surface of the ground in ancient periods. 



334 GEIKIE 

and were subsequently buried deep beneath sedimentary accumula- 
tions now removed, on the other hand we find masses of igneous 
rock which certainly never came near the surface, but must have 
been arrested in their ascent from below while still at a great depth, 
and have been laid bare to the light after the removal of the pile of 
rock under which they originally lay. 

By noting these and other characters, geologists have learnt that, 
besides the regions of still active volcanoes, there are few large areas 
of the earth's surface where proofs of former volcanic action or of 
the protrusion of igneous rocks may not be found. The crust of the 
earth, crumpled and fissured, has been, so to speak, perforated and 
cemented together by molten matter driven up from below. 

{b) Metamorphic. — The sedimentary rocks of the land have 
undergone many changes since their formation, some of which are 
still far from being satisfactorily accounted for. One of these changes 
is expressed by the term Metamorphism, and the rocks which have 
undergone this process are called Metatnorphic. It seems to have 
taken place under widely varied conditions, being sometimes con- 
fined to small local tracts, at other times extending across a large 
portion of the continent. It consists in the rearrangement of the com- 
ponent materials of rocks, and notably in their recrystallisation along 
particular lines or laminae. It is usually associated with evidence of 
great pressure; the rocks in which it occurs having been corrugated 
and crumpled, not only in vast folds, which extend across whole 
mountains, but even in such minute puckerings as can only be 
observed with the microscope. It shows itself more particularly 
among the older geological formations, or those which have been 
once deeply buried under more recent masses of rock, and have been 
exposed as the result of the removal of these overlying accumulations. 
The original characters of the sandstones, shales, grits, conglomer- 
ates, and limestones, of which, no doubt, these metamorphic masses 
once consisted, have been more or less effaced, and have given place 
to that peculiar crystalline laminated or foliated structure so dis- 
tinctively a result of metamorphism. 

An attentive examination of a metamorphic region shows that 
here and there the alteration and recrystallisation have proceeded so 
far that the rocks graduate into granites and other so-called igneous 
rocks. A series of specimens may be collected showing unaltered or 



GEOGRAPHICAL EVOLUTION 335 

at least quite recognisable sedimentary rocks at the one end, and 
thoroughly crystalline igneous rocks at the other. Thus the remark- 
able fact is brought home to the mind that ordinary sandstones, 
shales, and other sedimentary materials may, in the course of ages, 
be converted by underground changes into crystalline granite. The 
framework of the land, besides being knit together by masses of 
igneous rock intruded from below, has been strengthened by the 
welding and crystallisation of its lowest rocks. It is these rocks 
which rise along the central crests of mountain chains, where, after 
the lapse of ages, they have been uncovered and laid bare, to be 
bleached and shattered by frost and storm. 

ii. — The Architecture of the Land. 

Let us now proceed to consider how these materials, sedimentary 
and crystalline, have been put together, so as to constitute the solid 
land of the globe. 

It requires but a cursory examination to observe that the sedi- 
mentary masses have not been huddled together at random; that, 
on the contrary, they have been laid down in sheets one over the 
other. An arrangement of this kind at once betokens a chronological 
sequence. The rocks cannot all have been formed simultaneously. 
Those at the bottom must have been laid down before those at the 
top. A truism of this kind seems hardly to require formal statement. 
Yet it lies at the very foundation of any attempt to trace the geo- 
logical history of a country. Did the rocks everywhere lie undis- 
turbed one above another as they were originally laid down, their 
clear order of succession would carry with it its own evident interpre- 
tation. But such have been the changes that have arisen, partly from 
the operation of forces from below, partly from that of forces acting 
on the surface, that the true order of a series of rocks is not always 
so easily determined. By starting, however, from where the succes- 
sion is normal and unbroken, the geologist can advance with confi- 
dence into regions where it has been completely interrupted; where 
the rocks have been shattered, crumpled, and even inverted. 

The clue which guides us through these labyrinths is a very simple 
one. It is afforded by the remains of once living plants and animals 
which have been preserved in the rocky framework of the land 
Each well-marked series of sedimentary accumulations contains its 



336 GEIKIE 

own characteristic plants, corals, crustaceans, shells, fishes, or other 
organic remains. By these it can be identified and traced from 
country to country across a whole continent. When, therefore, the 
true order of superposition of the rocks has been ascertained by ob- 
serving how they lie upon each other, the succession of their fossils 
is at the same time fixed. In this way the sedimentary part of the 
earth's crust has been classified into different formations, each char- 
acterised by its distinct assemblage of organic remains. In the most 
recent formations, most of these remains are identical with still living 
species of plants and animals; but as we descend in the series and 
come into progressively older deposits the proportion of existing 
species diminishes until at last all the species of fossils are found to 
be extinct. Still lower and older rocks reveal types and assemblages 
of organisms which depart farther and farther from the existing 
order. 

By noting the fossil contents of a formation, therefore, even in a 
district where the rocks have been so disturbed that their sequence 
is otherwise untraceable, the geologist can confidently assign their 
relative position to each of the fractured masses. He knows, for 
instance, using for our present purpose the letters of the alphabet to 
denote the sequence of the formations, that a mass of limestone con- 
taining fossils typical of the formation B must be younger than 
another mass of rock containing the fossils of A. A series of strata 
full of the fossils of H resting immediately on others charged with 
those of C, must evidently be separated from these by a great gap, 
elsewhere filled in by the intervening formations D, E, F, G. Nay, 
should the rocks in the upper part of a mountain be replete with the 
fossils proper to D, while those in the lower slopes showed only the 
fossils of E, F, and G, it could be demonstrated that the materials 
of the mountain had actually been turned upside down, for, as 
proved by its organic remains, the oldest and therefore lowest forma- 
tion had come to lie at the top, and the youngest, and therefore high- 
est, at the bottom. 

Of absolute chronology in such questions science can as yet give 
no measure. How many millions of years each formation may have 
required for its production, and how far back in time may be the era 
of any given group of fossils, are problems to which no answer, other 



GEOGRAPHICAL EVOLUTION 337 

than a mere guess, can be returned. But this is a matter of far less 
moment than the relative chronology, which can usually be accu- 
rately fixed for each country, and on which all attempts to trace back 
the history of the land must be based. 

While, then, it is true that most of the materials of the solid land 
have been laid down at successive periods under the sea, and that the 
relative dates of their deposition can be determined, it is no less 
certain that the formation of these materials has not proceeded unin- 
terruptedly, and that they have not finally been raised into land by 
a single movement. The mere fact that they are of marine origin 
shows, of course, that the land owes its origin to some kind of 
terrestrial disturbance. But when the sedimentary formations are 
examined in detail, they present a most wonderful chronicle of long- 
continued, oft-repeated, and exceedingly complex movements of the 
crust of the globe. They show that the history of every country has 
been long and eventful; that, in short, hardly any portion of the 
land has reached its present condition, save after a protracted series 
of geological revolutions. 

One of the most obvious and not the least striking features in the 
architecture of the land is the frequency with which the rocks, 
though originally horizontal, or approximately so, have been tilted 
up at various angles, or even placed on end. At first it might be sup- 
posed that these disturbed positions have been assumed at random, 
according to the capricious operations of subterranean forces. They 
seem to follow no order, and to defy any attempt to reduce them to 
system. Yet a closer scrutiny serves to establish a real connection 
among them. They are found, for the most part, to belong to great, 
though fractured, curves, into which the crust of the earth has been 
folded. In low countries far removed from any great mountain 
range, the rocks often present scarcely a trace of disturbance, or if 
they have been affected, it is chiefly by having been thrown into 
gentle undulations. As we approach the higher grounds, however, 
they manifest increasing signs of commotion. Their undulations be- 
come more frequent and steeper, until, entering within the moun- 
tain region, we find the rocks curved, crumpled, fractured, inverted, 
tossed over each other into yawning gulf and towering crest, like 
billows arrested at the height of a furious storm. 



338 GEIKIE 

Yet even in the midst of such apparent chaos it is not impossible 
to trace the fundamental law and order by which it is underlaid. 
The prime fact to be noted is the universal plication and crumpling 
of rocks which were at first nearly horizontal. From the gentle 
undulations of the strata beneath the plains to their violent con- 
tortion and inversion among the mountains, there is that insensible 
gradation which connects the whole of these disturbances as parts 
of one common process. They cannot be accounted for by any mere 
local movements, though such movements no doubt took place 
abundantly. The existence of a mountain chain is not to be explained 
by a special upheaval or series of upheavals caused by an expansive 
force acting from below. Manifestly the elevation is only one phase 
of a vast terrestrial movement which has extended over whole con- 
tinents, and has affected plains as well as high grounds. 

The only cause which, so far as our present knowledge goes, could 
have produced such widespread changes is a general contraction of 
the earth's mass. There can be no doubt that at one time our planet 
existed in a gaseous, then in a liquid condition. Since these early 
periods it has continued to lose heat, and consequently to contract 
and to grow more and more solid, until, as the physicists insist, it has 
now become practically as rigid as a globe of glass or of steel. But 
in the course of the contraction, after the solid external crust was 
formed, the inner hot nucleus has lost heat more rapidly than the 
crust, and has tended to shrink inward from it. As a consequence 
of this internal movement, the outer solid shell has sunk down upon 
the retreating nucleus. In so doing, it has of course had to accom- 
modate itself to a diminished area, and this it could only accomplish 
by undergoing plication and fracture. Though the analogy is not 
a very exact one, we may liken our globe to a shrivelled apple. The 
skin of the apple does not contract equally. As the internal moisture 
passes off, and the bulk of the fruit is reduced, the once smooth 
exterior becomes here and there corrugated and dimpled. 

Without entering into this difficult problem in physical geology, it 
may suffice if we carry with us the idea that our globe must once 
have had a greater diameter than it now possesses, and that the 
crumpling of its outer layers, whether due to mere contraction or, as 
has been suggested, to the escape also of subterranean vapours, 



GEOGRAPHICAL EVOLUTION 339 

aflfords evidence of this diminution. A little reflection suffices to 
show us that, even without any knowledge of the actual history of 
the contraction, we might anticipate that the effects would neither 
be continuous nor everywhere uniform. The solid crust would not, 
we may be sure, subside as fast as the mass inside. It would, for a 
time at least, cohere and support itself, until at last, gravitation prov- 
ing too much for its strength, it would sink down. And the areas 
and amount of descent would be greatly regulated by the varying 
thickness and structure of the crust. Subsidence would not take place 
everywhere; for, as a consequence of the narrower space into which 
the crust sank, some regions would necessarily be pushed up. These 
conditions appear to have been fulfilled in the past history of the 
earth. There is evidence that the terrestrial disturbance has been 
renewed again and again, after long pauses, and that, while the 
ocean basins have on the whole been the great areas of depression, 
the continents have been the lines of uprise or relief, where the rocks 
were crumpled and pushed out of the way. Paradoxical, therefore, 
as the statement may appear, it is nevertheless strictly true, that the 
solid land, considered with reference to the earth's surface as a 
whole, is the consequence of subsidence rather than of upheaval. 

Grasping, then, this conception of the real character of the move- 
ments to which the earth owes its present surface configuration, we 
are furnished with fresh light for exploring the ancient history and 
growth of the solid land. The great continental ridges seem to lie 
nearly on the site of the earliest lines of relief from the strain of 
contraction. They were forced up between the subsiding oceanic 
basins at a very early period of geological history. In each succeeding 
epoch of movement they were naturally used over again, and received 
an additional push upward. Hence we see the meaning of the evi- 
dence supplied by the sedimentary rocks as to shallow seas and 
proximity of land. These rocks could not have been otherwise pro- 
duced. They were derived from the waste of the land, and were 
deposited near the land. For it must be borne in mind that every 
mass of land as soon as it appeared above water was at once attacked 
by the ceaseless erosion of moving water and atmospheric influences, 
and immediately began to furnish materials for the construction of 
future lands to be afterwards raised out of the sea. 



340 GEIKIE 

Each great period of contraction elevated anew the much-worn 
land, and at the same time brought the consolidated marine sedi- 
ments above water as parts of a new terrestrial surface. Again a 
long interval would ensue, marked perhaps by a slow subsidence 
both of the land and sea bottom. Meanwhile the surface of the land 
was channelled and lowered, and its detritus was spread over the 
sea floor, until another era of disturbance raised it once more with 
a portion of the surrounding ocean bed. These successive upward 
and downward movements explain why the sedimentary formations 
do not occur as a continuous series, but often lie each upon the 
upturned and worn edges of its predecessors. 

Returning now to the chronological sequence indicated by the 
organic remains preserved among the sedimentary rocks, we see 
how it may be possible to determine the relative order of the suc- 
cessive upheavals of a continent. If, for example, a group of rocks, 
which as before may be called A, were found to have been upturned 
and covered over by undisturbed beds C, the disturbance could be 
affirmed to have occurred at some part of the epoch represented else- 
where by the missing series B. If, again, the group C were observed 
to have been subsequently tilted, and to pass under gently-inclined 
or horizontal strata E, a second period of disturbance would be 
proved to have occurred between the time of C and E. 

I have referred to the unceasing destruction of its surface which 
the land undergoes from the time when it emerges out of the sea. 
As a rule, our conceptions of the rate of this degradation are exceed- 
ingly vague. Yet they may easily be made more definite by a con- 
sideration of present changes on the surface of the land. Every 
river carries yearly to the sea an immense amount of sand and mud. 
But this amount is capable of measurement. It represents, of course, 
the extent to which the general level of the surface of the river's 
drainage basin is annually lowered. According to such measure- 
ments and computations as have been already made, it appears that 
somewhere about -grcnnr of a foot is every year removed from the 
surface of its drainage basin by a large river. This seems a small 
fraction, yet by the power of mere addition it soon mounts up to 
a large total. Taking the mean level of Europe to be 600 feet, its 
surface, if everywhere worn away at what seems to be the present 



GEOGRAPHICAL EVOLUTION 34 1 

mean normal rate, would be entirely reduced to the sea-level in 
little more than three and a half millions of years. 

But of course the waste is not uniform over the whole surface. It 
is greatest on the slopes and valleys, least on the more level grounds. 
A few years ago, in making some estimates of the ratios between the 
rates of waste on these areas, I assumed that the tracts of more rapid 
erosion occupy only one-ninth of the whole surface affected, and 
that in these the rate of destruction is nine times greater than on the 
more level spaces. Taking these proportions, and granting that a.o'^ao 
of a foot is the actual ascertained amount of loss from the whole 
surface, we learn by a simple arithmetical process that t^ of an 
inch is carried away from the plains and tablelands in seventy-five 
years, while the same amount is worn out of the valleys in eight and 
a half years. One foot must be removed from the former in 10,800 
years, and from the latter in 1,200 years. Hence, at the present rate 
of erosion, a valley 1,000 feet deep may be excavated in 1,200,000 
years — by no means a very long period in the conception of most 
geologists. 

I do not offer these figures as more than tentative results. They 
are based, however, not on mere guesses, but on data which, though 
they may be corrected by subsequent inquiry, are the best at present 
available, and are probably not far from the truth. They are of value 
in enabling us more vividly to realise how the prodigious waste of 
the land, proved by the existence of such enormous masses of sedi- 
mentary rock, went quietly on age after age, until results were 
achieved which seem at first scarcely possible to so slow and gentle 
an agency. 

It is during this quiet process of decay and removal that all the 
distinctive minor features of the land are wrought out. When first 
elevated from the sea, the land doubtless presents on the whole a 
comparatively featureless surface. It may be likened to a block of 
marble raised out of the quarry — rough and rude in outline, massive 
in solidity and strength, but giving no indication of the grace into 
which it will grow under the hand of the sculptor. What art effects 
upon the marble block, nature accomplishes upon the surface of the 
land. Her tools are many and varied — air, frost, rain, springs, tor- 
rents, rivers, avalanches, glaciers, and the sea — each producing its 



342 GEIKIE 

own characteristic traces in the sculpture. With these implements, 
out of the huge bulk of the land she cuts the valleys and ravines, 
scoops the lake-basins, hews with bold hand the colossal outlines of 
the mountains, carves out peak and crag, crest and cliff, chisels the 
courses of the torrents, splinters the sides of the precipices, spreads 
out the alluvium of the rivers, and piles up the moraines of the 
glaciers. Patiently and unceasingly has this great earth-sculptor sat 
at her task since the land first rose above the sea, washing down into 
the ocean the debris of her labour, to form the materials for the 
framework of future countries; and there will she remain at work so 
long as mountains stand, and rain falls, and rivers flow. 

II. The Growth of the European Continent. 

Passing now from the general principles with which we have 
hitherto been dealing, we may seek an illustration of their applica- 
tion to the actual history of a large mass of land. For this purpose 
let me ask your attention to some of the more salient features in the 
gradual growth of Europe. This continent has not the simplicity of 
structure elsewhere recognisable; but without entering into detail or 
following a continuous sequence of events, our present purpose will 
be served by a few broad outlines of the condition of the European 
area at successive geological periods. 

It is the fate of continents, no less than of the human communities 
that inhabit them, to have their first origin shrouded in obscurity. 
When the curtain of darkness begins to rise from our primeval 
Europe, it reveals to us a scene marvellously unlike that of the 
existing continent. The land then lay chiefly to the north and north- 
west, probably extending as far as the edge of the great submarine 
plateau by which the European ridge is prolonged under the Atlantic 
for 230 miles to the west of Ireland. Worn fragments of that land 
exist in Finland, Scandinavia, and the northwest of Scotland, and 
there are traces of what seem to have been some detached islands in 
Central Europe, notably in Bohemia and Bavaria. Its original height 
and extent can of course never be known; but some idea of them 
may be formed by considering the bulk of solid rock which was 
formed out of the waste of that land. I find that if we take merely 
one portion of the detritus washed from its surface and laid down in 



GEOGRAPHICAL EVOLUTION 343 

the sea — viz. that which is comprised in what is termed the Silurian 
system — and if we assume that it spreads over 60,000 square miles of 
Britain with an average thickness of 16,000 feet, or 3 miles, which is 
probably under the truth, then we obtain the enormous mass of 
180,000 cubic miles. The magnitude of this pile of material may be 
better realised if we reflect that it would form a mountain ridge 
three times as long as the Alps, or from the North Cape to Marseilles 
(1800), with a breadth of more than 33 miles, and an average height 
of 16,000 feet — that is, higher than the summit of Mont Blanc. All 
this vast pile of sedimentary rock was worn from the slopes and 
shores of the primeval northern land. Yet it represents but a small 
fraction of the material so removed, for the sea of that ancient time 
spread over nearly the whole of Europe eastwards into Asia, and 
everywhere received a tribute of sand and mud from the adjoining 
shores. 

There is perhaps no mass of rock so striking in its general aspect 
as that of which this northern embryo of Europe consisted. It lacks 
the variety of composition, structure, colour, and form, which dis- 
tinguishes rocks of more modern growth; but in dignity of massive 
strength it stands altogether unrivalled. From the headlands of the 
Hebrides to the far fjords of arctic Norway it rises up grim and 
defiant of the elements. Its veins of quartz, felspar, and hornblende 
project from every boss and crag like the twisted and knotted sinews 
of a magnificent torso. Well does the old gneiss of the north deserve 
to have been made the foundation-stone of a continent. 

What was the character of the vegetation that clothed this earliest 
prototype of Europe is a question to which at present no definite 
answer is possible. We know, however, that the shallow sea which 
spread from the Atlantic southward and eastward over most of 
Europe was tenanted by an abundant and characteristic series of 
invertebrate animals — trilobites, graptolites, cystideans, brachiopods, 
and cephalopods, strangely unlike, on the whole, to anything living 
in our waters now, but which then migrated freely along the shores 
of the arctic land between what are now America and Europe. 

The floor of this shallow sea continued to sink, until over Britain, 
at least, it had gone down several miles. Yet the water remained 
shallow because the amount of sediment constantly poured into it 



344 GEIKIE 

from the northwest filled it up about as fast as the bottom subsided. 
This slow subterranean movement was varied by uprisings here and 
there, and notably by the outburst at successive periods of a great 
group of active submarine volcanoes over Wales, the Lake district, 
and the south of Ireland; but at the close of the Silurian period a 
vast series of disturbances took place, as the consequence of which 
the first rough outlines of the European continent were blocked out. 
The floor of the sea was raised into long ridges of land, among 
which were some on the site of the Alps, the Spanish peninsula, and 
the hills of the west and north of Britain. The thick mass of marine 
sediment was crumpled up, and here and there even converted into 
hard crystalline rock. Large enclosed basins, gradually cut off from 
the sea, like the modern Caspian and Sea of Aral, extended from 
beyond the west of Ireland across to Scandinavia and even into the 
west of Russia. These lakes abounded in bone-covered fishes of 
strange and now long-extinct types, while the land around was 
clothed with a club moss and reed-like vegetation — Psilophyton, 
Sigillaria, Calamite, etc. — the oldest terrestrial flora of which any 
abundant records have yet been found in Europe. The sea, dotted 
with numerous islands, appears to have covered most of the heart 
of the continent. 

A curious fact deserves to be noticed here. During the convul- 
sions by which the sediments of the Silurian sea floor were crumpled 
up, crystallised, and elevated into land, the area of Russia seems to 
have remained nearly unaffected. Not only so, but the same immu- 
nity from violent disturbance has prevailed over that vast territory 
during all subsequent geological periods. The Ural Mountains on 
the east have again and again served as a line of relief, and have been 
from time to time ridged up anew. The German domains on the 
west have likewise suffered extreme convulsion. But the wide inter- 
vening plateau of Russia has apparently always maintained its flat- 
ness either as sea-bottom or as terrestrial plains. As I have already 
remarked, there has been a remarkable persistence alike in exposure 
to and immunity from terrestrial disturbance. Areas that lay along 
lines of weakness have suffered repeatedly in successive geological 
revolutions, while tracts outside of these regions of convulsion have 



GEOGRAPHICAL EVOLUTION 345 

simply moved gently up or down without material plication or 
fracture. 

By the time of the coal growths, the aspect of the European area 
had still further changed. It then consisted of a series of low ridges 
or islands in the midst of a shallow sea or of wide salt-water lagoons. 
A group of islands occupied the site of some of the existing high 
grounds of Britain. A long, irregular ridge ran across what is now 
France from Brittany to the Mediterranean. The Spanish peninsula 
stood as a detached island. The future Alps rose as a long, low 
ridge, to the north of the eastern edge of which lay another insular 
space, where now we find the high grounds of Bavaria and Bohemia. 
The shallow waters that wound among these scattered patches of 
land were gradually silted up. Many of them became marshes, 
crowded with a most luxuriant cryptogamic vegetation, specially of 
lycopods and ferns, while the dry grounds waved green with conifer- 
ous trees. By a slow intermittent subsidence, islet after islet sank 
beneath the verdant swamps. Each fresh depression submerged 
the rank jungles and buried them under sand and mud, where they 
were eventually compressed into coal. To this united co-operation 
of dense vegetable growth, accumulation of sediment, and slow sub- 
terranean movement, Europe owes her coal-fields. 

All this time the chief area of high ground in Europe appears still 
to have lain to the north and northwest. The old gnarled gneiss of 
that region, though constantly worn down and furnishing materials 
towards each new formation, yet rose up as land. It no doubt 
received successive elevations during the periods of disturbance, 
which more or less compensated for the constant loss from its surface. 

The next scene we may contemplate brings before us a series of 
salt lakes, covering the centre of the continent from the north of 
Ireland to the heart of Poland. These basins were formed by the 
gradual cutting off of portions of the sea which had spread over the 
region. Their waters were red and bitter, and singularly unfavour- 
able to life. On the low intervening ridges a coniferous and cyca- 
daceous vegetation grew, sometimes in quantity sufficient to supply 
materials for the formation of coal seams. The largest of these salt 
lakes stretched from the edge of the old plateau of Central France 



346 GEIKIE 

along the base of the Alpine ridge to the high grounds of Bohemia, 
and included the basin of the Rhine from Bale down to the ridge 
beyond Mayence, which has been subsequently cut through by the 
river into the picturesque gorge between Bingen and the Siebenge- 
birge. This lake was filled up with red sand and mud, limestone, 
and beds of rock salt. Where the eastern Alps now rise the opener 
waters were the scene of a long-continued growth of dolomite, out 
of which in later ages the famous dolomite mountains of the Tyrol 
were carved. 

These salt lakes of the Triassic period seem to have been every- 
where quietly effaced by a widespread depression, which allowed the 
water of the main ocean once more to overspread the greater part of 
Europe. This slow subsidence went on so long as to admit of the 
accumulation of masses of limestone, shale, and sandstone, several 
thousand feet in thickness, and probably to bring most of the insular 
tracts of Central Europe under water. To this period, termed by 
geologists the Jurassic, we can trace back the origin of a large part 
of the rock now forming the surface of the continent, from the low 
plains of Central England up to the crests of the northern Alps, while 
in the Mediterranean basin, rocks of the same age cover a large area 
of the plateau of Spain, and form the central mass of the chain of 
the Apennines. It is interesting to know that the northwest of 
Britain continued still to rise as land in spite of all the geographical 
changes which had taken place to the south and east. We can trace 
even yet the shores of the Jurassic sea along the skirts of the moun- 
tains of Skye and Ross-shire. 

The next long era, termed the Cretaceous, was likewise more 
remarkable for slow accumulation of rock under the sea than for 
the formation of new land. During that time the Atlantic sent its 
waters across the whole of Europe and into Asia. But they were 
probably nowhere more than a few hundred feet deep over the site 
of our continent, even at their deepest part. Upon their bottom there 
gathered a vast mass of calcareous mud, composed in great part of 
foraminifera, corals, echinoderms, and molluscs. Our English chalk 
which ranges across the north of France, Belgium, Denmark, and 
the North of Germany, represents a portion of the deposits of that 
sea floor, probably accumulated in a northern, somewhat isolated 



GEOGRAPHICAL EVOLUTION 347 

basin, while the massive hippurite Hmestone of Southern Europe 
represents the deposits of the opener ocean. Some of the island spaces 
which had remained for a vast period above water, and had by their 
degradation supplied materials for the sediment of successive geo- 
logical formations, now went down beneath the Cretaceous sea. The 
ancient high-grounds of Bohemia, the Alps, the Pyrenees, and the 
Spanish tableland were either entirely submerged, or at least had 
their area very considerably reduced. The submergence likewise 
affected the northwest of Britain; the western highlands of Scotland 
lay more than 1,000 feet below their present level. 

When we turn to the succeeding geological period, that of the 
Eocene, the proofs of widespread submergence are still more striking. 
A large part of the Old World seems to have sunk down; for we find 
that one wide sea extended across the whole of Central Europe and 
Asia. It was at the close of this period of extreme depression that 
those subterranean movements began to which the present con- 
figuration of Europe is mainly due. The Pyrenees, Alps, Apennines, 
Carpathians, the Caucasus, and the heights of Asia Minor mark, as it 
were, the crests of the vast earth-waves into which the solid frame- 
work of Europe was then thrown. So enormous was the contortion 
that, as may be seen along the northern Alps, the rocks for thousands 
of feet were completely inverted, this inversion being accompanied 
by the most colossal folding and twisting. The massive sedimentary 
formations were crumpled up, and doubled over each other, as we 
might fold a pile of cloth. In the midst of these commotions the 
west of Europe remained undisturbed. It is strange to reflect that 
the soft clays and sands under London are as old as some of the 
hardened rocks which have been upheaved into such picturesque 
peaks along the northern flanks of the Alps. 

After the completion of these vast terrestrial disturbances, the 
outlines of Europe began distinctly to shape themselves into their 
present form. The Alps rose as a great mountain range, flanked on 
the north by a vast lake which covered all the present lowlands of 
Switzerland, and stretched northward across a part of the Jura 
Mountains, and eastward into Germany. The size of this fresh-water 
basin may be inferred from the fact that one portion only of the 
sand and gravel that accumulated in it even now measures 6,000 feet 



348 GEIKIE 

in thickness. The surrounding land was densely clothed with a 
vegetation indicative o£ a much warmer climate than Europe now 
can boast. Palms o£ American types, as well as date palms, huge 
Californian pines (Sequoia), laurels, cypresses, and evergreen oaks, 
with many other evergreen trees, gave a distinctive character to the 
vegetation. Among the trees too were planes, poplars, maples, wil- 
lows, oaks, and other ancestors of our living woods and forests; 
numerous ferns grew in the underwood, while clematis and vine 
wound themselves among the branches. The waters were haunted 
by huge pachyderms, such as the dinotherium and hippopotamus; 
while the rhinoceros and mastodon roamed through the woodlands. 

A marked feature of this period in Europe was the abundance 
and activity of its volcanoes. In Hungary, Rhineland, and Central 
France, numerous vents opened and poured out their streams of 
lava and showers of ashes. From the south of Antrim, also, through 
the west coast of Scotland, the Faroe Islands, and Iceland, even far 
into arctic Greenland, a vast series of fissure-eruptions poured forth 
successive floods of basalt, fragments of which now form the exten- 
sive volcanic plateaux of these regions. 

The mild climate indicated by the vegetation in the deposits of the 
Swiss lake prevailed even into Polar latitudes, for the remains of 
numerous evergreen shrubs, oaks, maples, walnuts, hazels, and many 
other trees have been found in the far north of Greenland, and even 
within 8° 15' of the pole. The sea still occupied much of the low- 
lands of Europe. Thus it ran as a strait between the Bay of Biscay 
and the Mediterranean, cutting off the Pyrenees and Spain from the 
rest of the continent. It swept round the north of France, covering 
the rich fields of Touraine and the wide flats of the Netherlands. 
It rolled far up the plains of the Danube, and stretched thence east- 
ward across the south of Russia into Asia. 

By this time some of the species of shells which still people the 
European seas had appeared. So long have they been natives of our 
area that they have witnessed the rise of a great part of the continent. 
Some of the most stupendous changes which they have seen have 
taken place in the basin of the Mediterranean, where, at a com- 
paratively recent geological period, parts of the sea floor were up- 
heaved to a height of 3,00a feet. It was then that the breadth of the 



GEOGRAPHICAL EVOLUTION 349 

Italian peninsula was increased by the belt of lower hills that flanks 
the range of the Apennines. Then, too, Vesuvius and Etna began 
their eruptions. Among these later geographical events also we must 
place the gradual isolation of the Sea of Aral, the Caspian, and the 
Black Sea from the rest of the ocean, which is believed to have once 
spread from the arctic regions down the west of Asia, along the 
base of the Ural Mountains into the southeast of Europe. 

The last scene in this long history is one of the most unexpected 
of all. Europe, having nearly its present height and outlines, is 
found swathed deep in snow and ice. Scandinavia and Finland are 
one vast sheet of ice, that creeps down from the watershed into the 
Atlantic on the one side, and into the basin of the Baltic on the other. 
All the high grounds of Britain are similarly buried. The bed of the 
North Sea as well as of the Baltic is in great measure choked with 
ice. The Alps, the Pyrenees, the Carpathians, and the Caucasus send 
down vast glaciers into the plains at their base. Northern plants find 
their way south even to the Pyrenees, while the reindeer, musk-ox, 
lemming, and their arctic companions, roam far and wide over 
France. 

As a result of the prolonged passage of solid masses of ice over 
them, the rocks on the surface of the continent, when once more 
laid bare to the sun, present a worn, flowing outline. They have 
been hollowed into basins, ground smooth, and polished. Long 
mounds and wide sheets of clay, gravel, and sand have been left 
over the low grounds, and the hollows between them are filled with 
innumerable tarns and lakes. Crowds of boulders have been perched 
on the sides of the hills and dropped over the plains. With the advent 
of a milder temperature the arctic vegetation gradually disappeared 
from the plains. Driven up step by step before the advancing flora 
from more genial climates, it retired into the mountains, and there 
to this day continues to maintain itself. The present Alpine flora 
of the Pyrenees, the Alps, Britain, and Scandinavia, is thus a living 
record of the ice age. The reindeer and his friends have long since 
been forced to return to their northern homes. 

After this long succession of physical revolutions, man appears as 
a denizen of the Europe thus prepared for him. The earliest records 
of his presence reveal him as a fisher and hunter, with rude flint- 



350 GEIKIE 

pointed spear and harpoon. And doubdess for many a dim century 
such was his condition. He made no more impress on external 
nature than one of the beasts which he chased. But in course of time, 
as civilisation grew, he asserted his claim to be one of the geographi- 
cal forces of the globe. Not content with gathering the fruits and 
capturing the animals which he found needful for his wants, he 
gradually entered into a contest with nature to subdue the earth and 
to possess it. Nowhere has this warfare been fought out so vigorously 
as on the surface of Europe. On the one hand, wide dark regions of 
ancient forest have given place to smiling cornfields. Peat and moor 
have made way for pasture and tillage. On the other hand, by the 
clearance of woodlands the rainfall has been so diminished that 
drought and barrenness have spread where verdure and luxuriance 
once prevailed. Rivers have been straitened and made to keep their 
channels, the sea has been barred back from its former shores. For 
many generations the surface of the continent has been covered with 
roads, villages and towns, bridges, aqueducts and canals, to which this 
century has added a multitudinous network of railways, with their 
embankments and tunnels. In short, wherever man has lived, the 
ground beneath him bears witness to his presence. It is slowly cov- 
ered with a stratum either wholly formed by him or due in great 
measure to his operations. The soil under old cities has been increased 
to a depth of many feet by the rubbish of his buildings; the level of 
the streets of modern Rome stands high above that of the pavements 
of the Csesars, and that again above the roadways of the early 
Republic. Over cultivated fields his potsherds are turned up in 
abundance by the plough. The loam has risen within the walls of 
his graveyards as generation after generation has mouldered into 
dust. 

It must be owned that man, in much of his struggle with the 
world around him, has fought blindly for his own ultimate interests. 
His contest, successful for the moment, has too often led to sure and 
sad disaster. Stripping forests from hill and mountain, he has gained 
his immediate object in the possession of their abundant stores of 
timber; but he has laid open the slopes to be parched by drought, or 
swept bare by rain. Countries once rich in beauty, and plenteous 
in all what was needful for his support, are now burnt and barren, 



GEOGRAPHICAL EVOLUTION 35 1 

or almost denuded of their soil. Gradually he has been taught, by his 
own bitter experience, that while his aim still is to subdue the earth, 
he can attain it, not by setting nature and her laws at defiance, but 
by enlisting them in his service. He has learnt at last to be the 
minister and interpreter of nature, and he finds in her a ready and 
unrepining slave. 

In fine, looking back across the long cycles of change through 
which the land has been shaped into its present form, let us realise 
that these geographical revolutions are not events wholly of the 
dim past, but that they are still in progress. So slow and measured 
has been their march, that even from the earliest times of human 
history they seem hardly to have advanced at all. But none the less 
are they surely and steadily transpiring around us. In the fall of 
rain and the flow of rivers, in the bubble of springs and the silence 
of frost, in the quiet creep of glaciers and the tumultuous rush of 
ocean waves, in the tremor of the earthquake and the outburst of 
the volcano, we may recognise the same play of terrestrial forces by 
which the framework of the continents has been step by step evolved. 
In this light the familiar phenomena of our daily experience acquire 
an historical interest and dignity. Through them we are enabled to 
bring the remote past vividly before us, and to look forward hope- 
fully to that great future in which, in the physical not less than in 
the moral world, man is to be a fellow-worker with God.